Portuguese representative Jose Filipe Moraes Cabral, left, and South African representative Baso Sangqu, right, glance at Russian representative Vitaly Churkin, center, as they vote in support of a draft resolution backing an Arab League call for Syrian President Bashar Assad to step down. (Jason DeCrow/AP)

Russia and China on Saturday vetoed a U.N. resolution condemning Syria’s violent repression of anti-government demonstrators, effectively quashing efforts to isolate President Bashar al-Assad’s government as it intensifies a nearly year-long crackdown.

The veto dealt a blow to attempts by the United States and its European partners to rally behind an Arab League plan that would require Assad to yield power and make way for a democratically elected unity government. The vote followed weeks of negotiations in which diplomats had significantly watered down the resolution in a bid to win broad support.

“The United States is disgusted” by the Russian and Chinese vetoes, Susan E. Rice, the U.S. ambassador to the United Nations, said after the vote. “A couple of members of this council remain steadfast in their willingness to sell out the Syrian people and shield a craven tyrant.”

Russia’s U.N. envoy, Vitaly I. Churkin, countered that the United States and its partners had undermined the prospects for a deal, saying they had promoted a strategy aimed at “regime change” by backing the opposition’s pursuit of power and fueling “armed methods of struggle.”

The rift left the diplomatic process in disarray, with Arab League delegates vowing to press ahead with their plan for a political transition in Syria, while Russia announced that officials would travel Tuesday to Damascus, where they will meet with Assad and try to push a competing plan to bring the Syrian government and the opposition together for direct talks.

But some Syria experts were worried that it was already too late for diplomatic solutions. “Things are slipping out of control on the ground so much that I’m not sure that [the resolution] could have stopped the killing,” said Andrew Tabler, a Middle East expert at the Washington Institute for Near East Policy.

The U.N. deadlock came a day after Syrian authorities moved to crush resistance in the town of Homs, killing scores of civilians on the 30th anniversary of the massacre in Hama. Estimates of those killed late Friday varied widely, but the assault seemed to be the strongest attempt yet by the government to put down the protests. Although casualties have been heavy for months, Syrian forces have largely abstained from the use of heavier weaponry. Activists now worry that the attack heralds a new and more aggressive strategy on the part of Assad’s government.

On Saturday, crowds gathered in Homs for the first funerals of the dead, with tens of thousands shown in video footage massing around coffins and shrouds decked with flowers. An opposition spokesman said that after the funerals, people were waiting eagerly to hear the results of the U.N. vote.

“We were hoping they would change their opinion,” said the spokesman, who uses the nom de guerre Abu Rami. “Unfortunately they used their veto. The people here are not so much disappointed. We will rely on Allah, the holy God, and after Allah, we will rely on the Free Syrian Army.”

The 13 to 2 Security Council vote capped weeks of tumultuous negotiations that pitted the United States, the European Union and the Arab League against Russia, Syria’s most powerful remaining protector in the 15-member council.

Secretary of State Hillary Rodham Clinton and Rice, along with their European and Arab counterparts, pushed repeatedly to avoid the Russia veto, agreeing to abandon the sharpest provisions in a draft resolution, including calls on states to prevent the supply of weapons to Syria, and to reinforce a set of Arab League sanctions on Damascus. They also added language that explicitly ruled out the use of the resolution as a pretext for future military action.

In Washington, President Obama issued a statement shortly before the vote casting Syria’s latest military operations as the last gasp of a crumbling regime and urging Assad to step down.

Envoys from Russia and China had signaled this past week that they would recommend their capitals support the U.N. resolution, according to senior council diplomats. But Russia’s foreign minister, Sergei Lavrov, ultimately rejected the compromise draft, proposing a set of last-minute amendments Saturday that were rejected by Washington and other sponsors of the resolution.

“We are not friends or allies of President Assad,” Lavrov insisted ahead of the vote. “We try to stick to our responsibilities as a permanent members of the Security Council, and the Security Council by definition does not engage in domestic affairs of member states.”

Senior administration officials pointed to the wide margin of support for the resolution, with developing countries such as Pakistan, India, South Africa and Azerbaijan all siding against the Assad government. “There was a hopeful aspect” to the vote, Rice said. “More and more countries are united in saying the violence must stop, change must come.”

White House officials acknowledged that the vote was a blow to diplomatic efforts to end the Syrian crisis. The focus now appears to shift to the Arab League, where a vote on sanctions by Arab nations could come later this month.

But critics of the administration’s incremental diplomacy in Syria said the administration was partly to blame for Saturday’s outcome.

“This has been a slow-motion train wreck,” said David Schenker, an adviser to the Pentagon on the Middle East during the George W. Bush administration. “We aimed low, and we fell short.”

Rather than attempting to sell the Russians on a toothless resolution, Schenker said, the White House should be mobilizing a “coalition of the willing” to help Syria’s beleaguered rebels with weapons and training. “If we continue to defer both to the Arab League and the U.N., the Syrian people are doomed.”

Saturday’s vote carried political risks for Moscow and Beijing, which have defied the wishes not only of Washington and its European powers, but confronted a coalition of influential and wealthy states likely to be driven even more deeply into the Western camp.

“There is nothing in this text that should have triggered a veto. We removed every possible excuse,” said Britain’s U.N. envoy, Mark Lyall Grant. “The reality is that Russia and China have today taken a choice: to turn their backs on the Arab world and to support tyranny rather than the legitimate aspirations of the Syrian people.”

In Syria, demonstrators were bracing for another possible onslaught.

Col. Malik Turki, a commander with the armed group known as the Free Syrian Army, said when reached by telephone in Turkey that he had received dozens of calls from Syrians asking him to supply them with weapons after the events of Friday night. He accused the authorities of pushing the country toward civil war and the international community of failing to stop government assaults.

“The Syrian citizens will do anything to get what is needed for their defense as the international community is giving this regime one chance after the other,” he said.

Fordham reported from Damascus. Staff writer Joby Warrick in Washington contributed to this report.