Western and Arab powers have reacted angrily to Russia and China's veto of a Security Council resolution on the Syria crisis, but Moscow and Beijing insisted the text had needed more work.

Russia, a long-time Syrian ally, and China had on Saturday vetoed a UN Security Council resolution condemning the Syrian government's deadly crackdown despite reports by Syrian activists that troops overnight had killed 230 civilians in the city of Homs.

US President Barack Obama denounced the "unspeakable assault" and demanded that Assad step down.

"Assad must halt his campaign of killing and crimes against his own people now.He must step aside and allow a democratic transition to proceed immediately," Obama said in a statement.

"Yesterday the Syrian government murdered hundreds of Syrian citizens, including women and children, in Homs through shelling and other indiscriminate violence, and Syrian forces continue to prevent hundreds of injured civilians from seeking medical help," the US president said.

"I strongly condemn the Syrian government's unspeakable assault against the people of Homs and I offer my deepest sympathy to those who have lost loved ones," he added.

Russia's UN ambassador Vitaly Churkin justified the veto by saying the proposed resolution "sent an unbalanced signal to the Syrian parties".

His Chinese counterpart Li Baodong said pushing through such "a vote when parties are still seriously divided ... will not help maintain the unity and authority of the Security Council, or help resolve the issue."

But the rest of the international community reacted with anger at the double veto, the second by the two countries since the start of the Syrian crisis a year ago.

UN leader Ban Ki-moon expressed deep regret, saying that it undermined the role of the United Nations, according to a statement.

French President Nicolas Sarkozy said the veto would encourage further crackdowns by the Syrian regime.

"The Syrian tragedy must stop," said Sarkozy in a statement issued through his office.

His Foreign Minister Alain Juppe said Moscow and Beijing "bear a heavy responsibility in the eyes of the world and the Syrian people" in vetoing the UN resolution, a move that he said "paralyses the international community".

His British counterpart William Hague said Russia and China had let the Syrian people down.

They had, he said, "sided with the Syrian regime and its brutal suppression of the Syrian people in support of their own national interests".

Italian Foreign Minister Giulio Terzi termed the double veto "very bad news", while Washington's US ambassador Susan Rice described the move by permanent Security Council members China and Russia as "shameful."

Russia and China "remain steadfast in their willingness to sell out the Syrian people and shield a craven tyrant," Rice told the council.

In a separate message on Twitter, she wrote: "Disgusted that Russia and China prevented the UN Security Council from fulfilling its sole purpose."

European Union chief diplomat Catherine Ashton also expressed regret at the vetoes.

"The time has come to speak with one voice and demand an end to the bloodshed and speak out for a democratic future for Syria," she said in a statement.

"We condemn the ongoing bloodshed and stand by the Syrian people against the repressive regime.

"We call on President Assad to end immediately the killing of civilians, withdraw the Syrian army from besieged towns and cities and step aside in order to make room for a peaceful transition for the sake of his country."

The European parliament expressed dismay and its president, Martin Schulz, urged Moscow and Beijing to "take their international responsibilities seriously".

London-based rights group Amnesty International called the veto a "shockingly callous betrayal" of the Syrian people.

Moscow and Beijing have acted in a "completely irresponsible" way, the London-based human rights group added.

Thirteen countries voted for the resolution with only Russia and China voting against. Both countries, as permanent members of the security council, have a veto power.

The draft resolution, put forward by Morocco, had called for an immediate end to all violence. It did not impose any sanctions, nor did it authorise military action.