Syria is on the edge of full-scale civil war following the latest violence in the country, United Nations secretary general Ban Ki-moon warned.
Speaking after a meeting of the UN Security Council in New York late on Thursday, he underlined the need for urgency in diffusing the situation in the country.
He said: "Syria can quickly go from a tipping point to a breaking point. The danger of full-scale civil war is imminent and real, with catastrophic consequences for Syria and the region."
There was little evidence of the Syrian regime complying with a peace plan to end the violence, he added, the Press Association reported.
Their comments came after reports that 78 people - including women and children - were killed in central Syria.
Observers from the UN were reportedly shot at as they tried to reach the scene of the massacre in central Hama.
Foreign secretary William Hague condemned the atrocity as "another example of the escalating horror and murder in Syria".
Mr Hague called for more action by Russia and China to press President Bashar Assad's regime to end the violence, saying the peace plan brokered by former UN secretary general Mr Annan had "clearly failed so far".
He said: "It is another example of what I would say is the escalating depravity and criminality of this regime."
Mr Annan yesterday urged the UN to unite behind efforts to end the Syrian conflict and called for "consequences" if his peace plan was not implemented, blaming its failure on President Bashar Assad's government.
He said: "Clearly, the time has come to determine what more can be done to secure implementation of the plan.
"We must also chart a clearer course for a peaceful transition, if we are to help the government and opposition, as well as Syrian society, to help resolve the crisis."
But Mr Hague said Mr Annan's plan wouldn't last indefinitely.
He said: Syria is clearly on the edge... of deeper violence, of deep, sectarian violence, village against village, pro-government militias against opposition areas and of looking more like Bosnia in the 1990s than of Libya last year."
He added: "The Annan plan has clearly failed so far but it is not dead, all hope for it is not lost."
Russia had "important leverage" over the Syrian regime, he added.
"If all the members of the Security Council and the whole Arab world increase the pressure on the Assad regime to implement that plan then it is still possible to do so," he said.
"It would take a big change on the part of the regime but time is not yet at an end. It's clearly running out because that violence is escalating, even violence against the regime appears to be escalating.
"We saw accounts of 80 Syrian soldiers killed the other day.
"So this is a rapidly deteriorating situation."
Mr Hague said the G20 summit later this month would provide an opportunity for the world to focus on the crisis in Syria.
"If the Annan plan does not work at all, if no-one, even then, is prepared to ensure that it is implemented, well, then we have to return to the UN Security Council to debate more robust and effective measures," he said.
Earlier yesterday, Prime Minister David Cameron called for concerted action by the international community against the Assad regime.
Speaking in Oslo, he said: "We need to do much more to isolate Syria, to isolate the regime, to put the pressure on and to demonstrate that the whole world wants to see a political transition from this illegitimate regime and to actually see one that can take care of its people.
"It really is appalling, what is happening in that country, and I want to see concerted action from the international community."