The spokesman for al-Qaeda's Syrian affiliate, al-Nusra Front, his son and 20 other fighters have been reported killed in air strikes in the northeast of the country.
Abu Firas al-Suri was meeting with other leading fighters in an al-Nusra stronghold, Kafar Jales, when the raids happened on Sunday, the Britain-based Syrian Observatory for Human Rights said.
Sources in the north of the country told Al Jazeera they believed the raids were carried out by the US-led coalition operating over Syria and Iraq. Other sources told the Reuters news agency the attacks appeared to have the hallmarks of US drone strikes.
A US security official said Washington was aware of reports on the killing but had no further information to offer on Sunday. Coalition forces have previously targeted al-Nusra Front leaders in Syria.
READ MORE: Al-Nusra Front arrests US-backed fighters in Syria
The Syrian Observatory for Human Rights had earlier said the strikes may have been launched by Syrian government or Russian fighter jets.
Al-Suri, whose real name was Radwan Nammous, fought against Soviet forces in Afghanistan when he met Al-Qaeda leader Osama bin Laden and his mentor Abdullah Azzam before returning to Syria after the uprising began in 2011.
A temporary ceasefire between government forces and rebels has largely held since February 27, but al-Nusra Front and the Islamic State of Iraq and the Levant group (ISIL) were not involved in that deal.
The cessation has, in fact, analysts say, allowed Russia and the US-led coalition that has been bombing ISIL, also known as ISIS, to concentrate their battle against the fighters.
On Wednesday, a drone strike near ISIL's de facto capital, Raqqa, killed Tunisian commander Abu al-Haija, according to the Observatory, the latest in a series of blows to the group in recent weeks.
Fifteen ISIL commanders accused of revealing his position have since been executed, the Observatory said, and the fate of another 20 men accused of collaborating with the US-led coalition is unknown.
On Sunday, the Syrian army seized the city of al-Qaryatain, one of the last ISIL strongholds in central Syria, according to state television.
Source: Al Jazeera and agencies