Updated

Russia and China vetoed a United Nations resolution Saturday calling on Syrian President Bashar Assad to step down, despite fresh appeals by President Obama and other world leaders following the deadly assault by Syrian forces on the city of Homs.

Activists say Syrian forces killed more than 200 people in what may be the bloodiest confrontation of the uprising against Assad's regime. Leading up to the U.N. Security Council meeting, Obama condemned the "unspeakable assault" and called on other nations to support the Arab League-backed resolution.

The other 13 members, including the U.S., France and Britain, supported the resolution. But Russia and China, which both have veto power, blocked the measure. Russia's Foreign Minister Sergey Lavrov had said the resolution made too few demands of anti-government armed groups, and that Moscow remains concerned that it could prejudge the outcome of a national dialogue among political forces in Syria.

U.S. Ambassador Susan Rice said America is "disgusted" by the vote.

"For months this council has been held hostage by a couple of members," Rice said.

French Ambassador Gerard Araud said Russia and China had "made themselves complicit in a policy of repression carried out by the Assad regime."

The vote forces diplomats at the U.N. to try to work out what could be a more watered-down version, despite the new wave of violence in Syria.

The Homs assault sparked fierce international outcry ahead of Saturday's meeting. Obama, in perhaps his most forceful statement to date about the unrest in Syria, accused the Assad regime of having "murdered hundreds of Syrian citizens, including women and children" in Homs. He called on Assad to step down, describing the violence as a sign of the regime's "inevitable collapse."

"Assad has no right to lead Syria, and has lost all legitimacy with his people and the international community," Obama said in a written statement released Saturday morning.

Ahead of the meeting, he called on the Security Council to act on "this abhorrent brutality," and take a stand against Assad.

The violence coincided with the 30-year commemoration of the massacre in the Syrian city of Hama by Assad's father, Hafez.

"Thirty years after his father massacred tens of thousands of innocent Syrian men, women, and children in Hama, Bashar al-Assad has demonstrated a similar disdain for human life and dignity," Obama said.

The U.N. says more than 5,400 people have been killed over almost 11 months in a government crackdown on civilian protests.

Tunisia decided to expel Syria's ambassador in response to the "bloody massacre" in Homs and no longer recognizes the Assad regime, the president's office said in a statement. Angry Syrians stormed their embassies in Berlin, London, Athens, Cairo and Kuwait city, clashing with guards and police and -- in Cairo -- setting fire to part of the embassy.

There were signs that the bombardment in Homs, Syria's third largest city, was in response to moves by army defectors to solidify control in several neighborhoods. There were reports that defectors set up new checkpoints in several areas, and two Homs activists said defectors attacked a military checkpoint in the Khaldiyeh district Thursday night, capturing 17 soldiers. The activists spoke on condition of anonymity to protect themselves from retaliation.

If defector activity was the spark, the assault signals a new willingness by the regime to unleash more devastating force against the dissidents. The defectors, part of a force called the Free Syrian Army, have grown increasingly bold in attacks on the military and attempts to take overt control in pro-opposition areas.

Khaldiyeh, a mainly Sunni neighborhood in the mixed city, took the brunt of the assault. Residents described a hellish night of ceaseless shelling that sent them fleeing to lower floors and basements of buildings.

"We were sitting at home and the mortars just started slamming into buildings around us," said Mohammad, a Khaldiyeh resident. "There was nothing that prompted it, not even protests ... people are terrified today," he added by telephone.

Mohammad, who like other Syrians in Homs declined to be further identified, said the shelling started shortly before midnight and lasted until the early morning hours Saturday. He said residents were out Saturday inspecting the damage, looking for relatives. "It's a catastrophe, no other way to describe it."

Online video by activists showed chaotic scenes in a makeshift clinic set up in what appeared to be a Khaldiyeh mosque, the room filled with wounded men with gashes and broken limbs being bandaged. Several dead bodies were shown. In another video, fire ravaged a house that had been shelled, as people poured water on the blaze.

The videos could not be independently verified.

The government denied the Homs bombardment and said corpses shown in videos were of people kidnapped by "terrorists" who filmed them to portray them as victims of alleged shelling.

Two main opposition groups, the British-based Syrian Observatory for Human Rights and the Local Coordination Committees, said the death toll in Homs was more than 200 people and included women and children. More than half of the deaths -- about 140 -- were in Khaldiyeh, they said.

The Syrian National Council, Syria's main opposition group, put the toll at more than 220.

"This is the worst attack of the uprising, since the uprising began in March until now," said Rami Abdul-Rahman, the head of the Observatory, which tracks violence through contacts on the ground.

Homs is a hotbed of dissent to Assad's regime and is known to shelter a large number of army defectors. Despite near daily regime raids and fighting, many parts of it remain outside of government control.

Assad has tried to crush the revolt with a sweeping crackdown since March. But neither the government nor the protesters are backing down. The opposition, which began with peaceful protests, has turned more and more to arms, and the military and security forces have responded with progressively greater force.

But the past weeks suggest the regime is moving to qualitiatively more powerful assaults. Last week, the military launched a heavy offensive in the suburbs east of Damascus after dissidents showed greater control there. Three days of fighting saw some of the highest daily death tolls of the uprising, until the regime appeared to silence the dissidents for now.

The Associated Press contributed to this report.