Air strikes killed at least four civilians in a truce zone outside the Syrian capital on Tuesday, the Syrian Observatory for Human Rights monitor said.
The Britain-based group said the deaths were the first in air strikes in the Eastern Ghouta region outside Damascus since a "de-escalation" zone was implemented there.
The toll was expected to rise because the strikes in the town of Douma had also left several people with serious injuries, the monitor added.
A safe zone agreed by Syrian regime allies Russia and Iran and rebel backer Turkey was implemented in parts of opposition-held Eastern Ghouta in late July.
The deal has seen violence drop off, though there has been sporadic fighting and shelling that have caused deaths.
The Observatory said it was unclear whether Tuesday's deadly strikes were carried out by Syrian government warplanes or those of its Russian ally.
Russia is a key sponsor of a plan to implement four "de-escalation zones" in Syria, including in Eastern Ghouta, southern Syria, central Syria and the northwest of the country.
It is also a strong supporter of Syrian President Bashar al-Assad and began a military intervention in the country in support of his government in 2015.
More than 330,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.
© 2017 AFP