BEIRUT (Reuters) - A bomb attack claimed by Islamic State killed more than 20 Syrian insurgents at a headquarters for the powerful Islamist Ahrar al-Sham group on Sunday, the Syrian Observatory for Human Rights monitoring group said.

Two blasts hit the base in a village east of Saraqeb in Idlib province, the British-based Observatory reported.

Ahrar al-Sham said in a statement a lone attacker had driven a motorbike up to the building, detonating explosives attached to himself and a bomb on the bike at the same time, killing and wounding dozens of rebels.

Photos on social media showed bloodied corpses and a blackened motorbike outside a small building in the village. The pictures could not immediately be verified.

Islamic State, which claimed the attack via its Amaq online news service, is opposed to all sides in Syria’s six-year civil war, including Syrian insurgent groups.

Idlib, a rebel stronghold, has also seen fighting between insurgent factions separate from battles with Islamic State.

Clashes earlier this year pitted groups aligned with Ahrar al-Sham against groups that joined the al Qaeda-linked Hayat Tahrir al-Sham alliance.

Syrian government forces have taken advantage of rebel infighting, particularly a separate spat further south near Damascus, to recapture territory from insurgents in the six-year conflict.