The Britain-based Syrian Observatory for Human Rights said 15 people died in strikes on the northern Syrian town of Manbij, which is under attack by predominantly Kurdish Syrian fighters, and six died in a strike on the nearby village of Tokhar.
The Local Coordination Committees, another activist group, also said 15 people, including women and children, died in Manbij.
The predominantly Kurdish Syria Democratic Forces have been trying to take control of Manbij from Daesh. The fighters have surrounded Manbij and are advancing slowly into the town.
Also Monday, Chris Gunness, spokesman for UN agency that supports Palestinian refugees, known as UNRWA, said a driver who works for the agency in Syria was killed after suffering a shrapnel wound in the northern city of Aleppo. Gunness said Yaser Mahmoud Shuaeeb, a 45-year-old father of six children, died Sunday.
UNRWA has 4,000 staff working in Syria. To date, 18 have perished as a result of the conflict and a further 25 are missing and or detained, Gunness said, the Associated Press reported.