A double suicide bombing at the headquarters of an ultra-conservative Islamic rebel group in northern Syria has killed its leader and several other high-ranking members.

The attack marked the second time in less than a year that a bombing has taken out the leadership of Ahrar al-Sham, one of the most powerful Islamic groups fighting both President Bashar Assad’s forces and its rival, Islamic State, which holds wide swathes of territory in Syria and Iraq.

The attack struck the headquarters of Ahrar al-Sham, northwest of the town of Idlib, killing the group’s leader, Abu Abdelrahman Salqeen.

The Syrian Observatory for Human Rights said six others Ahrar al-Sham members also died.

The Local Coordination Committees and other activists also reported the attack.

There was no immediate claim of responsibility for the bombing.