BEIRUT (AP) — Syria opposition activists say a suicide attack in rebel-controlled Idlib province killed at least five members of a powerful ultraconservative insurgent group.

It was not clear who was behind the Sunday attack. Northern Idlib is an opposition stronghold, but an al-Qaida-linked group has the most sway there and it is at odds with Ahrar al-Sham, the powerful ultraconservative group targeted in the attack.

Britain-based Syrian Observatory for Human Rights said at least five Ahrar al-Sham members were killed in the village of Tal Touqan and expected the death toll to rise. A leading member was killed in the attack, according to the Observatory.

Other opposition media platforms, such as Baladi News Network, quoted a witness who said at least 15 were killed in the attack that targeted a group meeting.