The leader of one of the biggest Islamist rebel groups in Syria has been killed by an explosion in the north-western province of Idlib.
Ahrar al-Sham said Hassan Abboud, pictured above, was among a number of senior figures who died in the blast at the group's headquarters in the town of Ram Hamdan.
There were conflicting reports about the cause of the blast and it was also unclear who was responsible.
IS has been battling rebel groups for control of northern and eastern Syria since January, when they launched a coordinated offensive to expel it from the country.
The explosion targeted a meeting of as many as 50 Ahrar al-Sham leaders in a basement at the group's heavily fortified headquarters.
The Islamic Front, a rebel umbrella group in which Ahrar al-Sham was the strongest faction, said that Abboud was among at least a dozen senior figures killed.
Abu Baraa, a member of a rebel group allied to Ahrar al-Sham said that a doctor who had examined the bodies said there was little visible sign of external injuries.
The doctor saw bodies with frothing at the mouth and fluid coming from the eyes and noses, Abu Baraa said, adding: "This was a highly sophisticated attack in a location that was very secure."
Ahrar al-Sham announced that Hashem al-Sheikh, known as Abu Jaber, would succeed Abboud as leader, while Abu Saleh Tahhan would be its new military chief.
"A group of the best leaders of Ahrar al-Sham have been martyred. But Ahrar al-Sham is more determined than ever to continue on the path to liberating our country from dictators," a video statement said.
IS is the most likely group to be blamed for the attack.
IS was accused of being behind a suicide bombing that killed another Ahrar al-Sham leader - Abu Khaled al-Suri, a veteran al-Qaeda operative - at his headquarters in Aleppo in February. al-Suri was Ayman Zawahiri's point man for al Qaeda in Syria. Zawahiri is the leader of al Qaeda.
In an interview before his death in June, Hassan Abboud denounced IS, saying it represented "the worst image ever of Islam."
Abboud, who was also known as Abu Abdullah al-Hamawi, was imprisoned by the Syrian authorities after taking part in the insurgency in Iraq but released in early 2011 as part of an amnesty.
He helped found Ahrar al-Sham (Free Men of the Levant) in Idlib province in late 2011 and in December 2012 formed the Syrian Islamic Front (SIF) with 10 other hardline Islamist groups.
Before its dissolution in November 2013, when the creation of the Islamic Front was announced, SIF had become the most powerful rebel force.
The Islamic Front refuses to come under the umbrella of the Western-backed Supreme Military Council (SMC) of the Free Syrian Army, but co-operates with SMC-aligned brigades on the battlefield, as well as the al-Nusra Front, al-Qaeda's affiliate in Syria.