The top military commander of the Army of Conquest, the largest Syrian rebel alliance, was killed in an airstrike during a meeting of the group's leaders, jihadist sources said Thursday.

The former Al-Nusra Front, renamed Fateh al-Sham Front, announced on Twitter "the martyrdom" of commander Abu Omar Sarakeb during an air strike in Syria's Aleppo province.

The Syrian Observatory for Human rights confirmed that airstrikes by unknown warplanes -- from the US-led coalition, Russia or the Syrian regime -- hit an Army of Conquest meeting in urban Aleppo in northern Syria and killed Omar Sarakeb and another military commander named as Abu Muslem al-Shami.

The Army of Conquest is the largest anti-regime rebel alliance in Syria, grouping Islamist factions including Ahrar al-Sham and Faylaq al-Sham with jihadists such as the Fateh al-Sham Front, previously Al-Qaeda affiliate Al-Nusra Front.

According to the Syrian Observatory, Abu Omar Sarakib was the rebel commander in last year's battle for control of Syria's northwestern Idlib province and a known Al-Qaeda in Iraq jihadist.

The Army of Conquest controls Idlib province, but last week suffered a major loss in southern Aleppo after a month-long rebel and jihadist offensive.