BEIRUT, Lebanon — The commander of one of the most powerful Syrian insurgent groups in the suburbs of the Syrian capital, Damascus, was killed Friday in an airstrike, according to the government and its opponents. The death of the commander, Zahran Alloush, is a significant blow to the armed opposition, bolstering President Bashar al-Assad ahead of a planned new round of peace talks.

Mr. Alloush led the Army of Islam, a group that had recently agreed to participate in a political process seeking to end the five-year-old conflict. The Army of Islam is regarded by the Syrian government and its most powerful ally, Russia, as a sectarian, terrorist group that differs little from more extremist groups like the Islamic State.

Local opposition figures reached in Damascus said the airstrikes had been carried out by Russian warplanes, but that information was not immediately confirmed by Russian or Syrian officials.

Analysts said the strikes were in keeping with longstanding efforts by the Syrian government and its allies to eliminate groups claiming to occupy a middle ground between Mr. Assad and the Islamic State. The efforts are part of a broader objective to improve Mr. Assad’s standing among Western governments, which despise him but also see the Islamic State as an increasing menace.