The Syrian Observatory for Human Rights said three children and a woman were among the dead in eastern Ghouta
An air strike killed eight civilians late on Monday in a rebel-held town east of Damascus where a truce was supposed to have taken effect at the weekend, a monitor said.
Three children and a woman were among the dead in the 11.30pm strike on Arbin in the eastern Ghouta rebel enclave where the government declared a ceasefire on Saturday, the Syrian Observatory for Human Rights said.
Observatory chief Rami Abdel Rahman said either government warplanes or those of its Russian ally carried out the strike, which also wounded at least 30 people.
The truce announced for eastern Ghouta on Saturday was the second unveiled under an agreement struck by government allies Iran and Russia and rebel backer Turkey in May.
A similar ceasefire was announced for southern areas of Syria on 9 July.
Under the agreement, so-called "de-escalation zones" are planned for four key battlegrounds between government forces and the rebels.
But 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.