A spokesman for the U.S.-backed Syrian forces and a war monitor say Islamic State militants have blown themselves up in the Syrian city of Raqqa, which liberated from the extremists nearly two years ago.

The Britain-based Syrian Observatory for Human Rights said Wednesday the militants clashed with the U.S-backed Syrian Democratic Forces and local security forces before blowing themselves up the night before.

SDF spokesman Mustafa Bali confirmed the report, saying that four IS members blew themselves up.

Raqqa, which the militants declared their de-facto capital, was liberated in October 2017, dealing a major blow to the group. The SDF and the U.S.-led coalition went on to fight the militants in rest of Syria, declaring military victory against IS last month. But Syria and Iraq are infested with sleeper cells.