World Bulletin/News Desk

Syrian rebel group Ahrar al-Sham has appointed a new leader and military chief after their predecessors were killed in a blast on Tuesday, a video statement from the group said.

The explosion in northwestern Syria killed at least 12 including Ahrar al-Sham's leader Hassan Aboud, according to the group, part of the Islamic Front alliance that has been fighting ISIL militants as well as the Syrian army.

The Syrian Observatory for Human Rights, a group that monitors the conflict, said an explosion killed 28 of Ahrar al-Sham's commanders, dealing a major blow to the group that is believed to have received funds from Gulf states.

In a video statement, Ahrar al-Sham said its council had appointed Hashem al-Sheikh, also known as "Abu Jaber", as leader and Abu Saleh Tahan as military chief. It pledged to keep fighting against Syrian government forces and ISIL.

The loss of senior leadership came at a significant time for Ahrar al-Sham, said Charles Lister, visiting fellow at the Brookings Doha Center.

"This potentially crippling blow comes during a period in which Ahrar al-Sham's senior officials have begun adopting more moderate stances in Syria, including considering joining larger moderate coalitions," he wrote in an analysis.

Some 50 of the group's leaders had been gathered at a house when the blast went off inside the meeting, according to the Observatory. There has been no claim of responsibility for the blast, which took place in Syria's Idlib province.

Some observers have described Tuesday's incident as a gas attack. Abu Baraa, a rebel figure from a group allied with Ahrar al-Sham, said a doctor who examined the bodies said there was little visible sign of external injuries.

Other reports suggested that the victims had died from smoke inhalation. It was not possible to independently verify any of the reports or pictures or the cause of the deaths.

The doctor saw bodies with frothing at the mouth and fluid coming from the eyes and noses, Abu Baraa said, adding the group had been meeting in was an heavily fortified underground bunker.

"This was a highly sophisticated attack in a location that was very secure," he said. It was not possible to independently verify any of the reports about the incident or the cause of the deaths.

In January another senior Ahrar al-Sham leader, Abu Khaled al-Soury, was killed in a suicide attack. Soury had fought alongside al Qaeda founder Osama bin Laden and was close to its current chief Ayman al-Zawahri.

ISIL denied involvement in that attack after being blamed for it.

Some supporters of ISIL celebrated Aboud's death on social media, saying that Syria had been "cleansed" of his presence.