At least eight people — including three children and a woman — were killed and 30 injured in an air strike in the rebel-held town east of Damascus, a British-based monitor said.
The strike on Arbin in Eastern Ghouta, where a ceasefire was declared on Saturday, took place at 11.30pm local time, said the Syrian Observatory for Human Rights.
Observatory chief Rami Abdel Rahman said that either the government or its Russian ally carried out the strike.
The truce announced in Eastern Ghouta was the second unveiled under an agreement struck by government allies Iran and Russia and rebel backer Turkey in May. On July 9, a ceasefire was announced for the southern areas of the country.
Under the agreement, so-called "de-escalation zones" are planned for four key battlegrounds between government forces and the rebels.
However, there have been disagreements over their borders and who will police them.
Russia said on Monday that it had set up two checkpoints and four observation posts in Eastern Ghouta to monitor the ceasefire.
* Additional reporting by Agence France-Presse
