The blast represents a boost for the Islamic State militant group, which has declared a caliphate on the Syria-Iraq border

The Islamic State received a fresh boost to its fortunes in Syria after an unexplained explosion wiped out the entire leadership of one of its most powerful rivals as it met to discuss a new strategy for taking on the jihadists.

Nearly fifty top leaders of Ahrar al-Sham were killed in the blast at an underground bunker in northwestern province, one of the few places left in Syria where mainstream opposition rebels are successfully holding the line against IS and the Assad regime.

Ahrar al-Sham, one of the oldest and largest armed opposition groups, is also the lynchpin of the Islamic Front, a coalition of anti-Assad Islamist militias formed with Saudi backing as a bulwark more extreme jihadist groups like IS. It was widely seen