The military leader of a key Islamist group in Syria has been killed in an air strike near the battleground city of Aleppo, dealing a major blow to the rebels trying to break a government siege.

Abu Hajer al Homsi, commander of Jabhat Fateh al-Sham, was killed on Thursday night in a rural province of Aleppo, where the group has played an instrumental role in the ongoing fight against forces loyal to President Bashar al-Assad.

The raid hit a meeting of senior leaders from different rebel groups, who had been discussing a joint counter-offensive to reopen the only route out of the opposition-held eastern side of the city.

Pro-regime forces this week managed to take back a strategically important district on Aleppo's southern outskirts, rolling back nearly every gain from a major month-long rebel offensive there and leaving the some 250,000 residents of the east blockaded again.