David Cameron today accused Syrian president Bashar Assad of endangering peace in the Middle East with his increasingly bloody crackdown on opponents.
The Prime Minister warned that the Assad regime's actions threatened to plunge Syria into full-scale civil war with dire consequences for the wider region.
Former United Nations secretary general Kofi Annan has been pushing Damascus to withdraw its troops from cities and halt all violence.
But yesterday's deadline passed without the regime honouring its commitments and activists were today reporting fresh violence in different parts of Syria.
Speaking at a press conference in Jakarta, Mr Cameron said: "We estimate that 1,000 people have died in the last week, 300 over the Easter weekend.
"Far from fulfilling their commitments, the regime is cynically exploiting the window of diplomatic negotiations to crack down even harder on its own people.
"With increasing refugee flows across international borders, Assad's actions are now threatening regional peace and security.
"We want to achieve a negotiated end to the Syrian crisis and avoid full-scale civil war. But Assad seems bent on doing precisely the opposite."
Mr Cameron urged the UN Security Council - and particularly Russia - to bear down on Syria.
He said he would be discussing the latest developments with Mr Annan later today.
"This is a decisive moment. The UN Security Council now has a clear responsibility to throw its full weight behind Kofi Annan's plan and to insist that it is implemented in full."