• Thirteen other council members vote in favour • UK and US react with fury to decision • Homs death toll more than 200, say activists

Russia and China have vetoed a UN security council resolution calling for the Syrian president to step down, provoking a furious reaction.

All 13 other members of the council, including the US, France and Britain, voted in favour of the resolution, which backed an Arab peace plan aimed at stopping the violence in Syria. Russia and China blocked the resolution because of what they perceived to be a potential violation of Syria's sovereignty, which could allow for military intervention or regime change.

William Hague, the foreign secretary, condemned the decision. "More than 2,000 people have died since Russia and China vetoed the last draft resolution in October 2011," he said after the vote. "How many more need to die before Russia and China allow the UN security council to act?

"Those opposing UN security council action will have to account to the Syrian people for their actions, which do nothing to help bring an end to the violence that is ravaging the country. The United Kingdom will continue to support the people of Syria and the Arab League to find an end to the violence and allow a Syrian-led political transition."

The draft resolution, tabled by Morocco, did not impose sanctions or authorise military action and contained nothing that warranted opposition, Hague said. Hillary Clinton, the US secretary of state, reacted angrily to the news at a press conference in Munich on Saturday night: "What more do we need to know to act decisively in the security council? To block this resolution is to bear responsibility for the horrors that are occurring on the ground in Syria."

Responding to the Russian foreign minister, Sergey Lavrov, who asked "What's the endgame?", Clinton replied: "The endgame in the absence of us acting together as the international community, I fear, is civil war."

Hague accused Russia and China of siding with "the Syrian regime and its brutal suppression of the Syrian people in support of their own national interests. Their approach lets the Syrian people down, and will only encourage President Assad's brutal regime to increase the killing, as it has done in Homs over the past 24 hours."

France's ambasador to the UN, Gerard Araud, said: "It is a sad day for the council. It is a sad day for Syria ... History has compounded our shame."

The defeat came despite concerted efforts by western leaders to get security council backing for the resolution censuring the Damascus regime.

Speaking before the vote, Barack Obama called for Assad to step down following the latest bloodshed. The US president said Assad had lost his legitimacy as a ruler and had "no right" to cling to power. He said the regime's policy of terrorising its people "only indicates its inherent weakness and inevitable collapse".

Britain and France also condemned the violence and called for decisive action by the international community in an apparent rebuke to Russia, which carried out its threat to veto the draft resolution.

Death tolls cited by activists and opposition groups ranged from 217 to 260, making the Homs attack the deadliest so far in Assad's crackdown on protests that erupted 11 months ago in response to uprisings that overthrew three Arab leaders.

Hague said it was time for countries to stop giving "shelter" to the regime after the assault on Homs. "The Syrian regime's actions display President Assad's cold-blooded cynicism in the face of mounting international pressure for the UN security council to do its utmost to end the bloodshed.

"The time is long past for the international community, particularly those that have so far sheltered the Assad regime, to intensify the pressure to end over 10 months of violence."

The French foreign minister, Alain Juppe, said the Homs bloodshed was a crime against humanity and "those who block the adoption of such a resolution are taking a grave historical responsibility".

But the Russian foreign minister, Sergey Lavrov, criticised the UN resolution, saying it made too few demands of anti-government armed groups, and could prejudge the outcome of a dialogue among political forces in the country.

Russian news agencies reported that Lavrov and Russia's foreign intelligence chief, Mikhail Fradkov, will meet with Assad in Damascus on Tuesday. Syria has been a key Russian ally since Soviet times and Moscow has opposed any UN demands that could be interpreted as advocating military intervention or regime change.

Earlier on Saturday, Tunisia decided to expel Syria's ambassador in response to the "bloody massacre" in Homs and said it no longer recognised the Assad regime. As news of the violence spread, a crowd of Syrians stormed their country's embassy in Cairo and protests broke out outside Syrian missions in Britain, Germany and the US.

Homs residents said pro-Assad forces began shelling the Khaldiya neighbourhood at around 8pm on Friday using artillery and mortars. They said at least 36 houses with families inside were destroyed. "We were sitting inside our house when we started hearing the shelling. We felt shells were falling on our heads," said Waleed, a resident of Khaldiya.

It was not immediately clear what had prompted Syrian forces to launch such an intense bombardment, just as diplomats at the security council were discussing the draft resolution supporting the Arab League demand for Assad to step aside.

Some activists said the violence was triggered by a wave of army defections in Homs, a stronghold of protests and armed insurgents whom Assad has vowed to crush. "The death toll is now at least 217 people killed in Homs, 138 of them killed in the Khaldiya district," Rami Abdulrahman, head of the British-based Syrian Observatory for Human Rights, told Reuters, citing witnesses.

"Syrian forces are shelling the district with mortars from several locations, some buildings are on fire. There are also buildings which got destroyed."

An activist said forces bombarded Khaldiya to scare other rebel neighbourhoods. "It does not seem that they get it. Even if they kill 10 million of us, the people will not stop until we topple him."

The opposition Syrian National Council said 260 civilians were killed, describing it as "one of the most horrific massacres since the beginning of the uprising in Syria". It said it believed Assad's forces were preparing for similar attacks around Damascus and in the northern town of Jisr al-Shughour.

Another group, the Local Co-ordination Committees, gave a death toll of more than 200. It is not possible to verify activist or state media reports as Syria restricts independent media access. Video footage on the internet showed at least eight bodies assembled in a room, one of them with the top half of its head blown off. A voice on the video said the bombardment was continuing as the video was being filmed.