11/30/2014 07:22 EST | Updated 01/30/2015 05:59 EST

U.S.-led coalition airstrikes pound ISIS stronghold in Syria

U.S.-led coalition warplanes carried out as many as 30 airstrikes overnight against ISIS militants in and around the group's de facto capital in northeastern Syria, activists said Sunday.

The Britain-based Syrian Observatory for Human Rights said the strikes targeted ISIS positions in the city of Raqqa as well as the Division 17 air base, which the militants seized earlier this year from government forces.

The monitoring group, which relies on a network of activists inside Syria, reported at least 30 coalition strikes in all. The Local Coordination Committees, an activist collective, also confirmed the airstrikes. Neither group had casualty figures.

There was no immediate confirmation from the U.S. military.

The American-led coalition began targeting ISIS militants in Syria in September, expanding an aerial campaign already hitting the extremist group in Iraq.