Al-Nusra Front is considered an affiliate of al-Qaeda in Syria.

 By Taraba Baraka

The military chief of Syria's al-Nusra Front militant group has been killed in an air strike, the group has said.

The jihadist group said on social media that three other leaders were killed along with Abu Homam al-Shami.

Syria's state-run news agency said the army had targeted Nusra leaders as they met in northern Idlib province, the Associated Press reported.

The Nusra Front is one of the most powerful groups fighting to overthrow Syrian President Bashar al-Assad.

It was involved in a major attack on the Air Force Intelligence headquarters in Aleppo on Wednesday.

Al-Nusra named the three other leaders killed as Abu Musab Falastini, Abu Omar Kurdi, and Abu Baraa Ansari.

Details of the attack are unclear. However, the official Syrian news agency described it as a "unique operation" carried out by the Syrian army in the al-Habit area.

Other sources quoted by Reuters said the strike had taken place in the town of Salqin, near the Turkish border.

Al-Nusra has emerged as the strongest rebel force around the embattled city of Aleppo.

Earlier on Thursday, fierce clashes took place in Aleppo near the air force facility attacked by Nusra Front rebels on Wednesday.

The Air Force Intelligence headquarters in Aleppo was badly damaged in an attack by the Nusra Front.

Government forces reportedly launched an assault on rebel positions in the west of the city.

Wednesday's attack began with the detonation of a large quantity of explosives in a tunnel under the building.

Nusra Front fighters and other hard-line rebel groups then launched a ground assault but were repelled by government forces.

Twenty soldiers and militiamen and 14 rebels were said to have been killed.

The US lists al-Nusra as a terrorist group and it is under UN Security Council sanctions.

The group has long been considered an affiliate of al-Qaeda, however there have been reports that it is considering cutting those ties to try to secure arms and finance from wealthy Gulf states.

Correspondents says those states, like the West, are looking for allies in the fight against Islamic State as well as against President Assad's forces.

The UN's envoy for Syria, Staffan de Mistura, has also hinted that the UN could negotiate with al-Nusra in order to allow the delivery of aid to civilians stranded in Aleppo.

He told the BBC's Lyse Doucet that he was willing to talk to everyone to save lives.

He said there were no talks now with al-Nusra but he hoped they would listen to his appeal, and there was a possibility he would talk to them too.