Originally published in BBC News
Hassan Abboud, the leader of the Syrian hardline Islamist rebel group Ahrar al-Sham, has been killed with other top commanders by a bomb blast.
A suicide bomber detonated an explosive vest at a meeting in the north-western town of Ram Hamdan, activists say.
Ahrar al-Sham is part of the Islamic Front, a powerful coalition of seven Islamist rebel groups.
In February, Ahrar al-Sham blamed the rival Islamic State (IS) for an attack which killed several of its members.
Activist groups say at least 40 people may have died in Tuesday’s attack, although the exact death toll is not clear.
Ahrar al-Sham is one of the biggest rebel groups seeking to overthrow President Bashar al-Assad.
Although the group is hardline Islamist in orientation, it has opposed and clashed with the ultra-radical Islamic State (IS), which is the most likely quarter to be blamed for the attack, reports the BBC Middle East correspondent Jim Muir.
IS was blamed when another Ahrar al-Sham leader, veteran al-Qaeda operative Abu Khaled al-Suri, was killed in a suicide bomb attack on his headquarters in Aleppo in February.
The death of Hassan Abboud comes at a moment when the US is seeking to unify the Syrian opposition into a co-ordinated fighting force against IS, although Ahrar al-Sham would have made an unlikely partner in such an American-led campaign, our correspondent says.
In an interview with the BBC in June, Abboud had denounced IS, saying it represented “the worst image ever of Islam”.
Abboud said that, unlike IS, Ahrar al-Sham would never pledge loyalty to outsiders and would only fight in Syria. It would continue fighting until Mr Assad’s government was ousted, he said.
He also stressed that the Islamic Front did not use suicide bombers and did not follow the “takfiri” doctrine, which holds that Muslim society has reverted to a state of unbelief, legitimising attacks on other Muslims.