BEIRUT — Islamic State militants blew themselves up following clashes with the U.S.-backed Syrian forces in the Syrian city of Raqqa, which was liberated from the extremists nearly two years ago, a war monitor said Wednesday.

The U.S-backed Syrian Democratic Forces first confirmed then denied the report.

The conflicting reports could not be immediately reconciled, and reflected the precarious security situation in the city, which the militants once called their capital, and which remains plagued with lawlessness despite efforts to rebuild.

SDF spokesman Mustafa Bali called the initial news a "misunderstanding." Bali said the explosion was caused by a land mine that wounded people. Another activist group, Raqqa Being Slaughtered Silently, said a land mine that went off in the western part of Raqqa killed two people.

But Rami Abdurrahman, head of Britain-based Syrian Observatory for Human Rights, said militants attacked a headquarters for the SDF in Raqqa city late Tuesday and early Wednesday, clashing with forces before blowing themselves up.

Abdurrahman said the Observatory said the explosions caused an undetermined number of casualties among the Syrian forces. He shared pictures of the killed militants, including one with a gutted belly.

Raqqa has been rife with lawlessness since it was liberated from the militants in October 2017.

Its liberation was a major blow to the militants. The SDF and the U.S.-led coalition went on to fight the militants in rest of Syria, declaring military victory against ISIS last month. But Syria and Iraq remain infested with sleeper cells.