Syrian opposition activists say airstrikes on villages and towns held by the Islamic State group in eastern Syria have killed and wounded dozens.

It was not immediately clear if the Saturday airstrikes on areas including Mayadeen, Boukamal, Bouleil, Bouomar and Mushassan were carried out by the Russians or the U.S.-led coalition.

The airstrikes came after two days of clashes between Syrian government forces and their allies against IS fighters in central and eastern Syria that left nearly 200 dead on both sides.

The Britain-based Syrian Observatory for Human Rights said the Saturday airstrikes and those the night before killed 18 people, including two children and five women.

Omar Abou Leila, of the monitoring group DeirEzzor 24, said 13 people were killed in Bouleil and nine others in Boukamal alone.