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. 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."
Russia had "important leverage" over the Syrian regime, he added.
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."