At least 16 senior members of the al-Nusra Front armed group have been killed in an air strike in Syria, according to an organisation that monitors the war there.
The strike hit a meeting the group was holding at the Abu al-Duhur airbase in the northwest of the country on Thursday, the Britain-based Syrian Observatory for Human Rights said.
One of the dead was a foreign commander, the monitor said.
It was not immediately clear who carried out the strike. Both Russia and the United States have previously targeted the al-Nusra Front in Syria.
Idlib province, where the attack took place, borders Turkey and is almost completely controlled by rebel groups, including the al-Nusra Front and Ahrar al-Sham.
Al-Nusra Front is part of an alliance of groups known as Jaish al-Fatah, which is fighting the forces of Syrian President Bashar al-Assad and his Russian- and Iranian-backed allies in the Aleppo countryside.
At least 250,000 people have been killed during Syria's five-year war, according to the United Nations, and four million people have been forced to flee the country.