The Syrian observatory for Human Rights said on Thursday that government air strikes killed at least 10 civilians in a key rebel bastion east of the capital, in violation of the truce.
Dozens more were wounded in the raids on Deir Al Assafir, a town in the opposition stronghold of Eastern Ghouta, added the observatory.
The toll, which included two children and a civil defense volunteer, represented "the largest ceasefire violation in Eastern Ghouta," Observatory head Rami Abdel Rahman said.
Syrian President Bashar al Assad had said "opposition forces" could form part of a new government, which would be agreed at peace talks in Geneva.
He told Russian news agency RIA he expected it to consist of "independent forces, opposition forces and forces loyal to the state".