Human Rights Watch (HRW) has accused both Moscow and Damascus of "extensive use" of banned cluster munitions in Syria.
HRW said it had documented the use of such weapons on at least 20 occasions since Syria and Russia began their joint offensive on September 30.
The New York-based rights group collected detailed information about attacks in nine locations that have killed at least 35 civilians, including five women and 17 children, and injured dozens. Two attacks hit camps for the displaced.
In a statement issued on December 20, HRW said all the recent cluster-munition attacks it had documented fell on opposition-controlled territory.
"Syria's promises on indiscriminate weapons ring hollow when cluster munitions keep hitting civilians in many parts of the country," said Ole Solvang, HRW's deputy emergencies director.
Cluster munitions are explosive weapons that can be delivered from the ground by artillery and rockets, or dropped from aircraft. A total of 118 countries have banned them.