A prominent leader in Syrian al Qaeda offshoot Nusra Front was killed on Sunday in what rebel sources said appeared to be a U.S. drone strike in the rebel-held northwestern province of Idlib.

The Syrian Observatory for Human Rights, which tracks violence in the country, confirmed reports on websites by militant sympathizers that Abu Firas, “the Syrian”, was killed along with a number of his companions.

While the Observatory said he was killed in a suspected Syrian or Russian air raid on a village northwest of the city of Idlib in northwestern Syria, two rebel sources said the attack appeared to have the hallmarks of a U.S. drone strike.

“The Sheikh was with his son and several companions,” said one of the rebel sources familiar with details of the incident.