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 the embattled city of Aleppo on Wednesday.
The group has long been considered an affiliate of al-Qaeda, although recent reports suggest it may be seeking to break those ties. The US lists it as a terrorist group and it is under UN Security Council sanctions.
'Unique operation'
News of the latest attack came as the UN's envoy for Syria, Staffan de Mistura, hinted to the BBC that the UN might negotiate with al-Nusra to allow much-needed aid to enter Aleppo.
Al-Nusra named the three other leaders killed on Thursday 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 Aleppo, Syria's second city.
Earlier on Thursday, fierce clashes took place in Aleppo near the air force facility attacked by Nusra Front rebels on Wednesday.
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.
Recent reports have suggested that al-Nusra is considering severing ties with al-Qaeda 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.
 de Mistura told the BBC's Lyse Doucet that he was willing to talk to everyone in an effort 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.