© REUTERS/Shaam News NetworkResidents carry the body of Khaled Saad Eddin, whom activists say was killed by forces loyal to Syria's President Bashar al-Assad, during his funeral at Houla, neighbourhood of Homs July 25, 2012. Picture taken July 25, 2012.
President Bashar al-Assad's artillery pounded rebel-held areas in and around Aleppo on Friday in preparation for an onslaught on Syria's biggest city where the United States has said it fears a "massacre" may be imminent.

Opposition sources said the shelling, which follows intensive ground and air bombardment, was an attempt to drive fighters inside Aleppo from their strongholds and to stop their comrades outside the city from resupplying them.

"They are shelling at random to instill a state of terror," said Anwar Abu Ahed, a rebel commander outside the city.

The battle for Aleppo, a major power centre that is home to 2.5 million people, is being seen as a potential turning point in the 16-month uprising against Assad that could give one side an edge in a conflict where both the rebels and the government have struggled to gain the upper hand.

A rebel commander said insurgents had attacked a convoy of Syrian army tanks heading towards the city, as the government continued to redeploy forces from other parts of the country to bolster its forces there.

The fate of Syria itself - an ethnically fragmented nation of 22 million people - is likely to determine the future of the wider region for years to come amid fears that its own sectarian tensions could spill across borders.

The U.S. State Department said credible reports of tank columns moving on Aleppo, along with air strikes by helicopters and fixed-wing aircraft, represented a serious escalation of Assad's efforts to crush his opponents.

"This is the concern: that we will see a massacre in Aleppo, and that's what the regime appears to be lining up for," Victoria Nuland, a spokeswoman for the State Department, said.

Turkey, a former ally of Assad and now one of his fiercest critics, cheered on the rebels in Aleppo.

"In Aleppo itself the regime is preparing for an attack with its tanks and helicopters ... my hope is that they'll get the necessary answer from the real sons of Syria," Prime Minister Tayyip Erdogan said in remarks broadcast on Turkish TV channels.

As the remaining residents of Aleppo braced themselves for more bloodshed, General Robert Mood, the outgoing head of the U.N. monitoring mission, told Reuters he thought Assad's days in power were numbered.

"In my opinion it is only a matter of time before a regime that is using such heavy military power and disproportional violence against the civilian population is going to fall," the Norwegian general, who left Damascus on July 19, said.

Navay Pillay, the United Nations human rights chief, said a pattern had emerged as Assad's forces resorted to shelling, tank fire and door-to-door searches.

"All this, taken along with the reported build-up of forces in and around Aleppo, bodes ill for the people of that city," she said in statement.

Mortar Rounds

Government troops stationed on the outskirts of the city unleashed barrages of heavy-caliber mortar rounds on its western districts, while Russian-built MI-25 helicopter gunships struck in the east, opposition activists inside the city said.

The heavy fighting follows an audacious bomb attack on July 18 that killed four of Assad's closest lieutenants in Damascus, a development that led some analysts to speculate that the government's grip was slipping.

In the first reported casualty on Friday, a man of about 60 wearing a traditional white prayer outfit, was killed near a park in Aleppo, while fighting spread across several neighborhoods.

A dawn bombardment killed five people who had been sheltering in a vegetable market. Video footage [no link provided] posted by opposition activists showed people gathering up the victims' body parts in plastic bags.

On Thursday, thirty-four people were killed in and around Aleppo, according to opposition activists, in an uprising that has cost the lives of 18,000 people across the country.

"The rebels have so far been nimble, and civilians have mostly been the victims of the bombardment," said activist Abu Mohammad al-Halabi, speaking by phone from the city.

Majed al-Nour, another activist, said rebels had attacked a security outpost in the neighborhood of Bustan al-Joz, which is close to Aleppo's city centre, on Thursday.

"The rebels are present in the east and west of the city, and have a foothold in areas of the centre. The regime forces control the entrances of Aleppo and the main thoroughfares and commercial streets and are bombarding the residential districts that fell into rebel hands," he said.

Nour said tens of thousands of people had fled Aleppo to nearby northern rural regions close to Turkey.

In the city, rebels have detained at least 100 Syrian officers, soldiers and pro-government militiamen this week, the Syrian Observatory for Human Rights, an opposition group, said.

A video posted on YouTube showed rebels with Kalashnikovs from "The Tawheed (monotheism) Brigade" guarding the detainees, who were lined up on a school playground.

Helicopters over Damascus

In Damascus on Friday, four helicopters flew over southern areas of the capital, firing heavy machine guns into the districts of Hajar al-Aswad and Tadamon as well as into the Yarmouk Palestinian refugee camp, a resident said.

"I can see two above me right now, heading towards Hajar al-Aswad," she said by telephone, the pounding of guns audible in the background. The helicopters were flying low and appeared to be targeting specific buildings.

Opposition sources said Syrian troops and armor entered Hajr al-Aswad on Friday, pursuing a counter-attack against rebel fighters that began last week.

With U.N. Security Council resolutions for sanctions against Syria vetoed by Russia and China for a third time last week, the United States has said it is stepping up assistance to Syria's fractured opposition, though it remains limited to non-lethal supplies such as communications gear and medical equipment.

Reuters has learned that the White House has crafted a presidential directive, called a "finding," that would authorize greater covert assistance for the rebels, but stop short of arming them.

It is unclear whether President Barack Obama has signed the document, a highly classified authorization for covert activity.

A Syrian parliamentarian, Ikhlas al-Badawi, from the northern province of Aleppo said on Friday she had fled to Turkey, becoming the first member of the rubber-stamp assembly dominated by Assad's Baath Party to defect.

Meanwhile, a source close to the mediation effort told Reuters on Friday that international mediator Kofi Annan was still trying to forge a political solution to the Syria crisis despite being made a scapegoat for the failure of the two sides to agree.

United Nations Secretary-General Ban Ki-moon said he was deeply concerned about reports that Syria could use chemical weapons and demanded that the government should state it would not use them "under any circumstances".

British Foreign Secretary William Hague said the Syrian government assault on Aleppo was an "utterly unacceptable escalation" of the conflict.

Additional reporting by Tabassum Zakaria, Matt Spetalnick and Andrew Quinn in Washington; Writing by Giles Elgood; Editing by Michael Roddy