Warplanes struck a key rebel bastion east of the Syrian capital for the first time since the fragile truce began last weekend, a monitor said.
"Two air strikes hit the edge of the town of Douma in Eastern Ghouta and one person was killed," said Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights.
He said either Syrian or Russian planes carried out the strikes.
Russia accused Turkey of continuing to bombard Kurdish positions and allowing jihadist groups to receive weapons across its border.
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"Such actions on the part of Turkey, aimed at supporting terrorist groups in the provinces of Aleppo and Idlib, put the ceasefire in danger," the Russian defence ministry said in a statement.
British, French, German and EU foreign ministers met in Paris to discuss the cessation of hostilities, saying there had been "real progress".
The group said the focus now was on convincing all parties to return to UN-brokered peace talks in Switzerland tentatively set for next Wednesday.
"We want a speedy resumption of the negotiations in Geneva, but two conditions must be fulfilled: access for all Syrians to humanitarian aid, and full respect of the ceasefire," said French Foreign Minister Jean-Marc Ayrault.
But after the group met with Riad Hijab, head of Syria's main opposition body, the High Negotiations Committee, he said the "conditions are not ripe" in Syria for a resumption of peace talks.
Hijab also said President Bashar al-Assad would have "no place" in a political transition because he has "blood on his hands".