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 and China 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 scores of civilians in the city of Homs.
Thirteen countries voted for the resolution proposed by European and Arab nations to give strong backing to the Arab League's plan to end the violence in Syria that has claimed thousands of lives across the country since March 2011.
Vitaly Churkin, the Russian ambassador to the UN, said the veto of the new resolution was necessary because it "sent an unbalanced signal to the Syrian parties".
Churkin said Western nations behind the resolution were "calling for regime change, pushing the opposition towards power".
Li Baodong, the Chinese representative to the UN, said further consultation was needed before the council denounced the Syrian government.
"To push through a vote when parties are still seriously divided over the issue will not help maintain the unity and authority of the Security Council, or help resolve the issue," Li said.
Hamadi Jebali, Tunisia's prime minister, on Sunday called for all countries to cut off diplomatic relations with Syria over the violence there.
"We have to expel Syrian ambassadors from Arab and other countries," Jebali said during a panel discussion on the Middle East at a security conference in the southern German city of Munich.
"The Syrian people do not expect from us today long statements ... they are expecting deeds, they are expecting concrete measures ... the very least we can do is to cut all relations to the Syria regime," added Jebali.
He said Russia and China's veto showed that the Security Council system was broken.
The veto was "a right that was misused and undoubtedly the international community has to reconsider this mechanism of decision taking," said Jebali.
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.
"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.
Ban Ki-moon, the UN secretary-general, said the double veto "is a great disappointment to the people of Syria and the Middle East and to all supporters of democracy and human rights".
"It undermines the role of the United Nations and the international community in this period when the Syrian authorities must hear a unified voice calling for an immediate end to its violence against the Syrian people," he said in 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.
The diplomatic developments come with activists reporting on Saturday that the Syrian army had killed 200 people since Friday, but this was later revised to 179.
The UK-based Syrian Observatory for Human Rights cited witnesses saying 138 of them in the Khaldiyeh neighbourhood of Homs.
In-depth coverage of escalating violence across Syria
Funerals were held on Sunday for some of those killed when Homs came under heavy bombardment.
The opposition Syrian National Council decried Saturday's violence as a "horrific massacre".
"The Syrian National Council calls on everyone around the world to speak up and do something to stop the bloodshed of innocent Syrians," it said in a statement.
The Local Co-ordination Committees, an opposition activist network, said more than 500 additional people were injured after the army used tanks, mortars and machine guns in the assault on the opposition stronghold.
Al Jazeera's Mysa Khalaf, reporting from Beirut, said sources in Syria told her the bombardment of the area started after the opposition Free Syrian Army, a group of army defectors, attacked Syrian army checkpoints and killed about 10 soldiers.
"I've been told that the main public hospital is completely overwhelmed and people have set up makeshift clinics in mosques. They are running low on supplies of blood," she said. "Several buildings have been destroyed."
The Syrian government denied the assault, saying the reports were part of a "hysterical campaign" of incitement by armed groups against Syria.