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 Syriaceasefire 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-basedSyrian 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.
State media and the Observatory said hundreds of people had left Wadi Barada in the past day for government-controlled areas nearby.
Earlier on Sunday government warplanes carried out several air strikes in the southern Aleppo countryside, the Observatory and rebel officials said.
Government forces also advanced overnight against rebels in the Eastern Ghouta area near Damascus, seizing 10 farms, the Observatory said.
A second rebel official suggested that low-level clashes on the ground would not necessarily derail the truce, but that air strikes were a "clear violation".
Russia's defence ministry has accused the insurgents in turn of violating the ceasefire numerous times.
A military news outlet run by Lebanese group Hezbollah, an ally of Assad, said the Syrian army had been targeting militants from the former Nusra Front both in southern Aleppo province and in Wadi Barada.
The army has said the group, previously al Qaeda's Syria branch, is not included in the ceasefire deal but rebels say it is - just one point of friction and confusion in the deal which could lead to its collapse.
The latest truce agreement is the first not to involve the United States or the United Nations - a reflection of Moscow's growing diplomatic influence after a long campaign of Russian air strikes helped Assad recapture the northern city of Aleppo last month.
That victory has greatly strengthened the president's position as the warring sides prepare for peace talks in the Kazakh capital Astana this month.