Foreign Secretary William Hague has warned that Syria is "clearly on the edge" of a descent into deeper violence following the latest massacre in the country.
The peace plan brokered by former UN secretary-general Kofi Annan has "clearly failed so far", Mr Hague said as he called for more action by Russia and China to press President Bashar Assad's regime to end the violence.
In a pessimistic assessment of the prospects of peace, he said: "Time is not yet at an end, it's clearly running out."
Violence on both sides is escalating and the situation is "rapidly deteriorating", he told reporters.
Mr Hague's comments came as UN Secretary-General Ban Ki-moon said observers from the mission in Syria were shot at as they tried to reach the scene of the massacre in central Hama.
The Foreign Secretary said the latest atrocity was "another example of the escalating horror and murder in Syria".
Kofi Annan spoke frankly at the New York meeting, at which 193 states were present
"It is another example of what I would say is the escalating depravity and criminality of this regime."
International envoy Mr Annan has urged the United Nations to unite behind efforts to end the conflict and called for "consequences" if his peace plan is not implemented.
Mr Hague said: "The Annan plan won't last indefinitely. 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."
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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.
"But, of course, any idea of such measures has been blocked in the past."
Russia and China had not shifted in their readiness to agree stronger security council resolutions, he said.
But he added they were both in favour of the Annan plan.
Taken from a video posted on the Facebook page for the Activists News Association this shows some of the bodies after the massacre. This has not been independently verified
"What we want is for them to join us in exercising their leverage, their power to ensure that the Assad regime actually does those things, actually implements the Annan plan."
He said Russia's refusal to back stronger action against the Assad regime had "given it the breathing space to fight on in this despicable, murderous way".
Earlier, 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."