Countries backing the Syria peace process are to meet in Geneva as France demanded information about attacks in breach of a cessation of hostilities that came into force early on Saturday.
"We have received indications that attacks, including by air, have been continuing against zones controlled by the moderate opposition," French Foreign Minister Jean-Marc Ayrault told reporters at the UN Human Rights Council in Geneva.
"All this needs to be verified. France has therefore demanded that the task force charged with overseeing the cessation of hostilities meet without delay."
A spokesman for the Saudi-backed opposition High Negotiations Committee (HNC) said the cessation of hostilities was broken by Syria's government 15 times within the first day, and that there were further violations by Russia and Hezbollah, both allies of President Bashar al-Assad.
The countries belonging to the "International Syria Support Group" (ISSG), led by the United States and Russia, are supposed to monitor compliance with the deal and act rapidly to end any flare-ups, while using force only as a last resort.
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But HNC spokesman Salim al-Muslat said it was still unclear how the system was supposed to function.
Asaad al-Zoubi, head of the HNC's delegation to peace talks, said on Monday that the cessation of hostilities had collapsed before it started and it faced "complete nullification", Al Arabiya al Hadath TV reported.
"We need to get an explanation from the Russians on the strikes that took place on Sunday," said one Western diplomat.
A Western ambassador in Geneva said he had always expected the cessation of hostilities would not be perfect.
"Everyone knew that there would be accidents," he told Reuters.
"But the ISSG will discuss it today. It's their view that will count."
