"Several units are transitioning out as several are transitioning in," said Lieutenant Colonel Mark Ballesteros, who said that as of Monday the number of US troops in Iraq was 150,000.

A Pentagon official, who asked not to be named, said the US Army's 4th Infantry Division was near the end of its year-long rotation.

US commanders in the past have timed the overlap of troop rotations to increase the US military presence during Iraqi elections and other critical milestones in the political process.

The increase this time follows the Muslim holy month of Ramadan, which US military officials anticipated would bring higher levels of violence, and comes just ahead of the November 7 congressional elections in the United States.

Pentagon officials, echoing Vice President Dick Cheney, have expressed concern in recent days that Al-Qaeda is trying to step up the violence in Iraq to influence the outcome of the US vote.

Cheney warned that deadly violence would plague Iraq for "some considerable period of time" and that it was up to Iraqis to decide how to tackle sectarian militias.

Asked about radical Iraqi cleric Moqtada al-Sadr and his Mahdi Army militia, Cheney told CNBC television that US-led forces had dealt with "the chief bad guy," Saddam Hussein, and that it was now up to Iraqis to handle Sadr.

Sadr "obviously speaks for a significant number of Iraqis, has a strong following. But if anything were to be addressed in that area, it's got to be addressed by the Iraqis themselves," Cheney said in the interview.

"The Iraqis are now the sovereign authority inside Iraq. That's their government. It's their responsibility. They'll have to make those judgments and decisions," he said.

Eric Ruff, the Pentagon press secretary, said he did not know why US troops levels were climbing.

"This is news to me," Ruff told reporters. "Talk to MNF-I (Multi-National Forces-Iraq). That's General Casey's decision."

The increase is noteworthy because US troop strength in Iraq is only 10,000 under the all-time high of about 160,000 reported in January after the Iraqi elections.

It had fallen to as low as 127,000 in June when US commanders still believed they could make troop cuts this year.

In late July, US Defense Secretary Donald Rumsfeld reversed directions and ordered a combat brigade to be extended beyond a year as part of buildup to quell burgeoning sectarian violence in Baghdad.

In mid-September, General John Abizaid, the top US commander in the Middle East, said more than 140,000 troops would be needed through the first six months of 2007 to check the violence.

Since then, US troop levels have oscillated between 142,000 to 147,000, the level it reached last week.

But the stepped-up operations have so far failed to bring the violence under control in Baghdad or stop its spread to other cities, prompting a review of military tactics.

General George Casey, the commander of US forces in Iraq, had said earlier this month that it was an "open question" whether bringing in more troops would have a lasting effect on the violence.

But he left the door open to it last week at a press conference in Baghdad.

"Now, do we need more troops to do that? Maybe," he said. "And as I've said all along, if we do, I will ask for the troops I need, both coalition and Iraqis."

Rumsfeld, however, insisted two days later that Casey's comments had been mischaracterized in press reports.

"Listen, we're in the political season," he said. "People are trying to take what he said and turn it in a way that it plays the way they'd like to see it play."

But she was left in little doubt of growing unhappiness and concern among MPs of all parties over the conduct of the Iraq war and the prospect there of a bloody civil war.

Adam Price, Welsh Nationalist MP for Carmarthen E and Dinefwr, opening the debate on a joint Welsh and Scottish Nationalist motion calling for an inquiry, said the "monumental catastrophe" of the Iraq war was the worst foreign policy disaster certainly since Suez, possibly since Munich.

The SNP and Plaid Cymru motion proposing a committee of seven senior MPs to review "the way in which the responsibilities of government were discharged in relation to Iraq", was defeated by 298 votes to 273, a Government majority of 25.

Mrs Beckett said there had already been four inquiries.

"So is this the moment to take a decision and a step of the kind recommended in the motion? My answer is a resounding no," she said.

She was challenged by Edward Leigh (C, Gainsborough) to promise an inquiry as soon as British troops left Iraq. To noisy heckling, she replied: "This is not the time for making these decisions."

She urged MPs to remember that what they said would be heard by British troops already in great danger in Iraq and by those whose intention it was "to do us harm, whether it be in Iraq or beyond." It was not the time to send a signal that many would interpret as a weakening of Britain's commitment. The Conservatives, who voted for the war three and a half years ago, called on the Government to agree to set up an inquiry by an independent committee of privy counsellors within a year.

It would be similar to the Franks inquiry carried out after the Falklands war, involving former generals and civil servants, not just MPs.

William Hague, Conservative foreign affairs spokesman, said he would be urging Tories to vote for the Nationalists' motion because of the Government's failure to promise any inquiry at all.

There was "a very strong case" for a wider Commons debate on Iraq. The military operation was "so vast, so expensive, so chequered with success and failures".

Mr Hague dismissed the argument that having a debate would harm troops' morale.

"The British Army is both tougher and more thoughtful than that and its operations should not be used as an excuse to avoid examining any of our political processes and judgments," he said.

Mr Price said there had been no comprehensive statement of Government policy on Iraq. "There are two Iraqs - the Iraq of George Bush and the Prime Minister, where things are going to plan and getting better all the time, and the real Iraq of murder and mayhem, whose future is uncertain," he said.

The state of denial which characterises the Government's policy now mirrors the state of delusion that characterised their policy in the run- up to war."

Sir Peter Tapsell (C, Louth and Horncastle) said the invasion and occupation of Iraq was a "strategic, political and humanitarian blunder of historic magnitude".

The Sunni insurgents seem to be following a plan to control all the approaches to Baghdad. They have long held the highway leading west to the Jordanian border and east into Diyala province. Now they seem to be systematically taking over routes leading north and south.

Dusty truck-stop and market towns such as Mahmoudiyah, Balad and Baquba all lie on important roads out of Baghdad. In each case Sunni fighters are driving out the Shia and tightening their grip on the capital. Shias may be in a strong position within Baghdad but they risk their lives when they take to the roads. Some 30 Shias were dragged off a bus yesterday after being stopped at a fake checkpoint south of Balad.

In some isolated neighbourhoods in Baghdad, food shortages are becoming severe. Shops are open for only a few hours a day. "People have been living off water melon and bread for the past few weeks," said one Iraqi from the capital. The city itself has broken up into a dozen or more hostile districts, the majority of which are controlled by the main Shia militia, the Mehdi Army.

The scale of killing is already as bad as Bosnia at the height of the Balkans conflict. An apocalyptic scenario could well emerge - with slaughter on a massive scale. As America prepares its exit strategy, the fear in Iraq is of a genocidal conflict between the Sunni minority and the Shias in which an entire society implodes. Individual atrocities often obscure the bigger picture where:

* upwards of 1,000 Iraqis are dying violently every week;

* Shia fighters have taken over much of Baghdad; the Sunni encircle the capital;

* the Iraqi Red Crescent says 1.5 million people have fled their homes within the country;

* the Shia and Sunni militias control Iraq, not the enfeebled army or police.

No target is too innocent. Yesterday a bomb tore through a party of wedding guests in Ur, on the outskirts of Sadr City, killing 15 people, including four children. Iraqi wedding parties are very identifiable, with coloured streamers attached to the cars and cheering relatives hanging out the windows.

Amid all this, Dick Cheney, the US Vice-President, has sought to turn the fiasco of Iraq into a vote-winner with his claim that the Iraqi insurgents have upped their attacks on US forces in a bid to influence the mid-term elections. There is little evidence to support this. In fact, the number of American dead has risen steadily this year from 353 in January to 847 in September and will be close to one thousand in October.

And there is growing confusion over the role of the US military. In Sadr City, the sprawling slum in the east of the capital that is home to 2.5 million people, American soldiers have been setting up barriers of cement blocks and sandbags after a US soldier was abducted, supposedly by the Mehdi Army. The US also closed several of the bridges across the Tigris river making it almost impossible to move between east and west Baghdad. Nouri al-Maliki, the Iraqi Prime Minister, added to the sense of chaos yesterday when he ordered the US army to end its Sadr City siege.

Mr Maliki has recently criticised the US for the failure of its security policy in Iraq and resisted American pressure to eliminate the militias. Although President Bush and Tony Blair publicly handed back sovereignty to Iraq in June 2004, Mr Maliki said: "I am now Prime Minister and overall commander of the armed forces yet I cannot move a single company without Coalition [US and British] approval."

In reality the militias are growing stronger by the day because the Shia and Sunni communities feel threatened and do not trust the army and police to defend them. US forces have been moving against the Mehdi Army, which follows the nationalist cleric Muqtada al-Sadr, but he is an essential prop to Mr Maliki's government. Almost all the main players in Iraqi politics maintain their own militias. The impotence of US forces to prevent civil war is underlined by the fact that the intense fighting between Sunni and Shia around Balad, north of Baghdad, has raged for a month, although the town is beside one of Iraq's largest American bases. The US forces have done little and when they do act they are seen by the Shia as pursuing a feud against the Mehdi Army.

One eyewitness in Balad said two US gunships had attacked Shia positions on Sunday killing 11 people and seriously wounding six more, several of whom lost legs and arms. He added that later two Iraqi regular army platoons turned up in Balad with little military equipment. When they were asked by locals why their arms were so poor "the reply was that they were under strict orders by the US commander from the [nearby] Taji camp not to intervene and they were stripped of their rocket-propelled grenade launchers".

Another ominous development is that Iraqi tribes that often used to have both Sunni and Shia members are now splitting along sectarian lines.

In Baghdad it has become lethally dangerous for a Sunni to wander into a Shia neighbourhood and vice versa. In one middle-class district called al-Khudat, in west Baghdad, once favoured by lawyers and judges, the remaining Shia families recently found a cross in red paint on their doors. Sometimes there is also a note saying "leave without furniture and without renting your house". Few disobey.

The Occupation: War and Resistance in Iraq by Patrick Cockburn is published this month by Verso

The abductions yesterday near the town of Tarmiyah marked a further outbreak of sectarian violence in a region where scores were killed last month in bloody attacks and reprisal killings among formerly friendly Shiite and Sunni neighbours in the city of Balad.

Unarmed men checked identification cards and seemed to be looking for familiar faces among travellers stopped in heavy traffic, said an eyewitness, who asked to be identified only by the pseudonym Abu Omar for fear of reprisals.

Armed gunmen stood nearby during the abductions, just out of sight of US soldiers who were disarming a roadside bomb further down the road, Abu Omar said. He and other Sunni travellers were allowed to travel onward after showing their ID cards, he said.

At least 40 travellers were missing and feared abducted, said an officer at the Joint Co-operation Centre in the city of Tikrit, 80 miles north of Baghdad.

Twelve victims of yesterday's attack on a Shiite wedding in Baghdad later died from their injuries, adding to the 11 killed in the blast, said Dr. Qassim al-Suwaidi from the al-Sadr hospital. Another 19 were still being treated at the hospital, he said.

The attack, in which a bomber drove an explosives-rigged car into a crowd outside the bride's home, was grimly similar to recent killings aimed at sparking Shiite retaliation and pushing Iraq toward all-out civil war - a stated goal of the al Qaida in Iraq extremist group.

Police said US and Iraqi forces last night stormed an office in the south-western hamlet of Ahrar belonging to the al-Sadr organisation, sponsors of the feared Mahdi Army militia linked to sectarian murders and other violence.

Senator Kerry, who lost to Mr Bush in the 2004 election, later said the line had been a "botched joke" aimed at the president; an aide explained that it had been intended to mean that Mr Bush's intellectual laziness had led the country into war but had been "mangled in delivery".
But the president and his party, in danger of losing Congress next Tuesday, sought to make the comment into the talking point of the campaign.

"Even in the midst of a heated campaign season, there are still some things we should all be able to agree on; and one of the most important is that every one of our troops deserves our gratitude and respect," Mr Bush said. "The senator's suggestion that the men and women of our military are somehow uneducated is insulting and shameful."

Senator Kerry hit back at what he called White House distortion, arguing it was "a remarkable testament to their abject failure in making America safe". The senator, a Vietnam war hero, said he rejected criticism from "Republican hacks, who have never worn the uniform of our country".

Some Democrats defended the senator, but others privately cringed. An unnamed Democratic congressman told ABC News: "I guess Kerry wasn't content blowing 2004, now he wants to blow 2006, too."

According to local press accounts of Monday's speech, Senator Kerry told students at Pasadena city college: "You know, education, if you make the most of it, you study hard and you do your homework, and you make an effort to be smart, you can do well. If you don't, you get stuck in Iraq."

The Kerry slip played into the White House's strategy of questioning the Democrats' national security credentials in the closing days of the elections. In a speech in Georgia on Monday, Mr Bush said that a Democratic win would hand victory to Iraqi insurgents. "However they put it, the Democrat approach in Iraq comes down to this: the terrorists win and America loses," he said. "The Democrat goal is to get out of Iraq. The Republican goal is to win in Iraq." Vice-president Dick Cheney told Fox News that the insurgents in Iraq were timing attacks according to the US election calendar.

"It's my belief that they're very sensitive of the fact that we've got an election scheduled," he said. "Whether it's al-Qaida, or the other elements that are active in Iraq, they are betting on the proposition that they can break the will of the American people."

The combative approach marks a shift in this election when Democrats have led the debate on Iraq, while Republicans have tried to avoid the topic.