© Sana handout/EPABashar al-Assad of Syria is facing invasion by NATO forces
EU leaders echo stinging rebuke, delivered by US president in executive order imposing sanctions and assets freeze

Barack Obama has led a demand by world leaders for the Syrian president, Bashar al-Assad, to surrender power. The US president declared that the "sustained onslaught" of Assad's regime against pro-democracy protesters has cost it all legitimacy.

The US president was joined by David Cameron of Britain, Nicolas Sarkozy of France and Angela Merkel of Germany, as well as the European Union in demanding Assad immediately resign.

Obama said the Syrian people's pursuit of democracy was an inspiration that had been met with "ferocious brutality" by their government.

"The future of Syria must be determined by its people, but President Bashar al-Assad is standing in their way. His calls for dialogue and reform have rung hollow while he is imprisoning, torturing, and slaughtering his own people," Obama said.

"We have consistently said that President Assad must lead a democratic transition or get out of the way. He has not led. For the sake of the Syrian people the time has come for President Assad to step aside."

David Cameron issued a joint statement with the President Sarkozy and Chancellor Merkel that noted Assad had ignored appeals from other Middle East states, the Arab League and the United Nations security council to end the violent crackdown.

"Our three countries believe that President Assad, who is resorting to brutal military force against his own people and who is responsible for the situation, has lost all legitimacy and can no longer claim to lead the country. We call on him to face the reality of the complete rejection of his regime by the Syrian people and to step aside in the best interests of Syria and the unity of its people."

The EU foreign policy chief, Catherine Ashton, said there had been a "complete loss of Bashar Assad's legitimacy in the eyes of the Syrian people".

Obama said the US was stepping up sanctions against Syria, including freezing its assets and banning petroleum products of Syrian origin, to pressure Assad from office. But he insisted that "the United States cannot and will not impose this transition upon Syria".

"It is up to the Syrian people to choose their own leaders, and we have heard their strong desire that there not be foreign intervention in their movement," Obama said.

"What the United States will support is an effort to bring about a Syria that is democratic, just and inclusive for all Syrians. We will support this outcome by pressuring President Assad to get out of the way of this transition, and standing up for the universal rights of the Syrian people along with others in the international community."

The US has calibrated its response to the violence in Syria, wary of Damascus's role as a strategic key to the Arab world and the risk the crisis could be exported beyond its borders.

Washington has also been cautious about putting its authority on the line, fearing damage to its standing if Assad were to defy its calls for him to go.

The call from western capitals came as it was revealed that UN human rights investigators have listed the names of 50 regime figures who could be prosecuted by the international criminal court (ICC) for crimes committed against civilians during the violent crackdown on pro-democracy demonstrators.

The list is believed to contain officials from the president's inner circle and security agencies. It marks the first time that government insiders have faced the prospect of criminal charges since the five-month uprising began.

A decision on whether to refer the names to the ICC is likely to be made on Thursday.

The UN report accuses officials of torture, summary executions and abuse of children - allegations that could amount to crimes against humanity. It accuses security forces of indiscriminately firing at demonstrators, sometimes from helicopters, and says injured protesters have been killed inside hospitals, sometimes being locked alive in mortuary freezers. It says Syrian officials confirmed that around 1,900 demonstrators had been killed by mid-July, and states that hundreds more have been killed since then.

"Children have not only been targeted by security forces, but they have been repeatedly subject to the same human rights and criminal violations as adults, including torture," the report said.

The report's authors were denied access to Syria and spent four months interviewing defectors and demonstrators who had fled the country.

Dozens of former members of the security forces have made their way to Amman and Istanbul, where they have detailed the orders given to them by senior officers to attack demonstrators.

Activists and defectors have compiled details of alleged atrocities by troops whose commanders insist they are targeting terrorists holding their communities to ransom.

The communities themselves have regularly given a diametrically opposed version of events, claiming that the armed men terrorising them are government-backed militias, known as al-shabiha or ghosts, who work with security forces.

One defector, a conscript who was deployed to the southern city of Dera'a in April, told the Guardian that his unit's first order was not to shoot at armed men. "The officer said they were with us," the soldier said. "They said we were only to shoot at the demonstrators."

In a telephone conversation on Wednesday night with the UN secretary general, Ban Ki-moon, Assad said the operations in the restive Syrian cities of Latakia and Homms had finished. However, activists on the ground reported on Wednesday that security forces were still active in both places.

In Latakia, a Mediterranean port city that has been the subject of a four-day military assault, security centres were overflowing with detainees and hundreds of prisoners were being held in the city's main football stadium and a cinema.

The push into Latakia ordered by commanders this week was stridently criticised by other nations in the region, with Saudi Arabia, Bahrain, Tunisia and Qatar withdrawing their ambassadors and Turkey warning it had uttered its "last words" on the crackdown.