Updated on December 26 at 12:41 p.m. ET

The Army of Islam, one of the main rebel groups fighting Syrian President Bashar al-Assad, has a new leader, a day after its chief was killed in an airstrike near Damascus, dealing a major blow to the groups that are fighting the Syrian military.

The group appointed Essam al-Buwaydhani, a field commander known as Abu Hammam, as its new leader, the Associated Press reports. He replaces Zahran Allouch, who was killed in Friday’s airstrike that was claimed by the Syrian government. Two other senior rebels, one from Ahrar al-Sham and the other from Faylaq al-Rahman, were also killed in the operation against the headquarters of the Army of Syria.

The deaths are a setback to the rebels groups that are fighting both Assad and the Islamic State group. Earlier this month, the Saudi-backed Army of Syria participated in a meeting of Syrian opposition groups in an attempt to choose a delegation to negotiate with the government’s representatives. Assad’s government regards the rebels as terrorists and had said it would not negotiate with them, though on Thursday it appeared to soften that stand. It’s unclear how Allouch’s death will affect either position.

Allouch, a former prisoner, was released by Assad in 2011 as part of a general amnesty. He joined the Syrian opposition and established the Army of Islam, which fast became one of the best organized groups. It controls large parts of eastern Ghouta and Douma, near Damascus, and is fighting both Assad as well as the Islamic State.