Activists say a double suicide bombing has targeted the headquarters of an ultraconservative Islamic rebel group in northern Syria, killing its leader and several other high ranking members.

The attacks northwest of the town of Idlib on Tuesday struck the headquarters of Ahrar al-Sham, 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 but Ahrar al-Sham is among the most powerful Islamic groups fighting both President Bashar Assad's forces and the Islamic State group, which holds wide swaths of territory in Syria.

It was the second time in less than a year that a bombing takes out Ahrar al-Sham's leadership.