Damascus: A UN-backed ceasefire to end 13 months of bloodshed in Syria appeared to be holding after coming into force at daybreak on Thursday, despite doubts about the regime's compliance with the peace plan.

After breaking a commitment to pull back forces from population centres by Tuesday under the agreement brokered by former UN chief Kofi Annan, there were still no signs of a withdrawal, monitors said.

"An hour after the ultimatum expired, the situation is calm in all regions," said Rami Abdel Rahman of the Britain-based Syrian Observatory for Human Rights.

A "few explosions" were heard in the town of Zabadani, just outside the capital, shortly after the ceasefire entered into effect, Abdel Rahman said in Beirut.

"There has not been any movement indicating a withdrawal of tanks," he added.

The report could not be verified due to the Damascus government's curbs on media.

Just hours before the deadline expired at 0830 IST, the military unleashed a lethal offensive against protest centres, killing 25 civilians on Wednesday, including 10 in the rebel stronghold of Rastan, said the Observatory.

That was the same day that Annan, serving as a peace envoy for the UN and the Arab League, said he received a letter from Syria promising to "cease all military fighting throughout Syrian territory as of 6:00 am."

But the letter reserved the right "to respond proportionately to any attacks carried out by armed terrorist groups," a phrase President Bashar al-Assad's government uses to describe opposition forces.

US President Barack Obama and German Chancellor Angela Merkel agreed in a telephone call that the UN Security Council, which gave its blessing to Annan's plan, needed to "take more resolute action" on Syria.