Blast kills nearly all the leaders of Syrian rebel group Ahrar al-Sham

An explosion of uncertain origin has killed nearly all the leaders of the largest rebel group fighting to topple the government of Syrian President Bashar al-Assad.

At least two dozen senior leaders of Ahrar al-Sham, a conservative Islamist group, died in the blast on Tuesday, which came 10 days after the group had distanced itself from al-Qaeda's official Syrian affiliate, the Nusra Front. The death toll, by some accounts, was as high as 75.

The rebel group hurriedly named new commanders. Hashim al-Sheikh Abu Jaber would replace Hassan Abboud as overall commander, the group announced. Abboud was also head of the Islamic Front's political bureau. There was no immediate indication of his replacement in that role.

Activists and witnesses gave varying versions of what took place at the former government agricultural research centre outside the town of Ram Hamdan near the Turkish border that had become a major Ahrar al-Sham base.

One account attributed the blast to a car bomb. But a senior member of Ahrar al-Sham, who tweets under the pseudonym Mujahid al-Sham, posted on Twitter that the explosion had originated in a workshop for manufacturing bombs that was adjacent to the room where the Ahrar al-Sham leaders were meeting. He said the explosion detonated huge amounts of TNT.

Syria analyst Charles Lister, writing for the Huffington Post, said that "the most likely scenario appears to be that a government air strike targeted the meeting, which was taking place in an underground bunker.

Only one survivor was reported, Allam Abboud, Hassan Abboud's younger brother. He was reportedly in a critical condition in hospital.