David Cameron urged Russia and China today to join the world in “tightening the noose” around the Syrian regime after a key deadline passed.
The Prime Minister said Bashar Assad's "deliberate flouting" of the latest peace plan showed that further action was needed.
Former United Nations secretary general Kofi Annan has secured commitments from Damascus to pull its troops out of cities and halt all violence.
But an initial deadline passed on Tuesday without the regime honouring its pledge to withdraw.
A ceasefire came into effect in the early hours of this morning, with reports so far suggesting it is being observed.
Speaking to BBC Radio 5 during his South East Asia trade mission today, Mr Cameron said: "I feel an immense sense of frustration because the world has come together behind this Kofi Annan plan.
"This is a plan, remember, that is not just backed by those of us who have been pushing for action on Syria, it's also backed by China and Russia.
"And yet Assad is deliberately flouting it.
"Now is the time to say to the Russians and Chinese, look at the man we are dealing with, look at the appalling way he is behaving.
"We need to go back to the UN and tighten the pressure, tighten the noose."
Syrian foreign minister Jihad Makdissi told BBC Radio 4's Today programme that President Assad's government had no intention of breaking the ceasefire and had already made several military drawbacks.
Mr Makdissi said the Syrian government was prepared to enter a process to end the conflict but said President Assad retained the support of a "majority" of Syrians.
He said: "There is no reason to break the ceasefire at all.
"What is important is everybody should abide by the terms of calm. (Kofi Annan) is asking cessation of violence by all parties, not only the Syrian government.
"For our side we are fully committed for this.
"We have a clear mechanism now in place, which is the Annan plan... we are enabling him.
"It is not about President Assad.... the realistic prospect is it is up to Syrians themselves to decide.
"Anybody who wants to change the regime in Syria, they should not be forcing this, they should do it through democratic channels.
"You have to be even-handed. The violence is mutual in Syria. I'm not saying we don't have problems in Syria.
"What I am saying is the way out is evolution and not armed confrontation."
Mr Makdissi said there could be no pre-conditions, such as the departure of President Assad, on any process.
The Prime Minister spoke to Kofi Annan earlier today and expressed concern that the April 10 deadline had not been observed, according to a Downing Street source.
Mr Annan told Mr Cameron that conditions on the ground seemed calm.
The former UN Secretary General's team will report to the Security Council this evening, and file a formal report on the situation on Monday.
Mr Cameron said it was a "good first step" if the ceasefire was proving effective. There is due to be a discussion on the issue at the Security Council next week.