An air strike killed eight civilians late on Monday in a rebel-held region near Damascus where a truce was declared at the weekend, medics and a monitoring group said.
The strike hit the town of Arbin in the Eastern Ghouta rebel enclave at around 11:30 pm (2030 GMT), residents said.
At least four children and one woman were among the dead, according to medics and the Syrian Observatory for Human Rights.
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 thrashed out under an agreement struck by government allies Iran and Russia and rebel backer Turkey in May.
The army said it would observe an indefinite freeze in fighting with rebels in the region.
But the pro-government Al-Watan newspaper reported on Monday that the truce excluded areas held by former Al-Qaeda affiliate Fateh al-Sham Front or its ally Faylaq al-Rahman -- which controls Arbin.
An AFP correspondent who visited the town's hospital early on Tuesday saw at least five small bodies laid out on the floor wrapped in white shrouds.
Two of them were toddlers.
"My sister came to visit me and she and her two children were killed," a woman cried.
A pair of exhausted doctors walked through the hospital after a long night treating patients.
The Observatory said there had been air strikes on Eastern Ghouta on Sunday, at sunset on Monday, and again on Tuesday morning, but that the deaths in Arbin were the first civilian deaths since the truce.
A similar ceasefire was announced for southern areas of Syria on July 9.
Under the May agreement, "de-escalation zones" are planned for four key battlegrounds between government and rebel forces, but there have been disagreements over their borders and who will police them.
The presence of jihadist and allied forces which control large parts of the remaining rebel enclaves has also complicated the ceasefire efforts.
The Syrian government and its Russian and Iranian allies insist that the jihadists cannot be included in any truce.
Russia said on Monday that it had set up two checkpoints and four observation posts in Eastern Ghouta to monitor the ceasefire.