Syrian government warplanes resumed their bombardment of a rebel-held valley near Damascus on Sunday after nearly 24 hours with no air raids, a rebel official and monitors said, during the third day of a fragile ceasefire.
The truce deal, brokered by Russia and Turkey which back opposing sides in the conflict and welcomed unanimously by the United Nations Security Council, has been repeatedly violated since it began, with warring sides trading the blame.
Rebels on Saturday warned they would abandon the truce if the government side continued to violate it, asking the Russians, who support President Bashar al-Assad, to rein in army and militia attacks in the valley by 8:00 p.m.
Bombardments ceased before that time - although some clashes continued - but began again late on Sunday.
It was not immediately clear if the rebels would abandon the truce as a result. Like previous Syria ceasefire deals, it has been shaky from the start with repeated outbreaks of violence in some areas, but has largely held elsewhere.
The raids hit areas of Wadi Barada, where government forces and their allies launched an operation more than a week ago, a spokesman for the Jaish al-Nasr rebel group and the British-based Syrian Observatory for Human Rights said.
There was a "fierce attack and attempt by Assad and Shi'ite militias to raid Wadi Barada" from nearby hills, the rebel spokesman, Mohammed Rasheed, said.