Air strikes, believed to have been carried out by U.S.-led forces, hit Islamic State and other Islamist groups in eastern Syria early on Saturday, a monitoring group said.
The Britain-based Syrian Observatory for Human Rights said at least 31 explosions were heard in the province of Raqqa, a stronghold of IS, and casualties were reported.
The monitoring group said warplanes also struck areas east of the desert town of Palmyra in Homs province.
The United States is leading a military coalition supported by some Gulf and European nations against Islamic State, a powerful force in Syria which swept across northern Iraq in June.
The campaign has brought Washington back to the battlefield inIraq that it left in 2011 and into Syria for the first time after avoiding involvement in a war that began the same year.
Militant groups in the region, and websites commonly used by supporters, did not immediately mention any attacks on Saturday.