BEIRUT (AP) — An attack on pro-government fighters by suspected members of the Islamic State group in central Syria killed 22 pro-government fighters, an opposition war monitor and pro-government media reported Friday.

Gunmen attacked a bus carrying members of the Quds Brigade, a government and Russian-backed faction of mostly Palestinian fighters in Syria, near the town of Sukhna late Thursday night. Sukhna was once an IS stronghold.

There was no immediate claim of responsibility but both the Syrian Observatory for Human Rights, a U.K.-based opposition war monitor, and the pro-government radio station Sham FM said IS was behind the attack.

Both the Observatory and Sham FM said 22 fighters were killed. Sham FM said they were all Quds Brigade gunmen, while the Observatory said the majority belonged to the group.

The Quds Brigade fought on the side of Syrian government forces during the country’s 13-year conflict, which has killed half a million people and displaced half the country’s pre-war population of 23 million.