Russian strikes Saturday on a prison complex run by Al-Qaeda's Syrian affiliate in the country's northwest killed at least 39 people, including five civilians, a monitoring group said.
The strikes hit an Al-Nusra Front building, which lies near a popular market in Maarat al-Numan in Idlib province, the Syrian Observatory for Human Rights said.
The building housed the group's religious court and a jail.
Most of those killed were rebels imprisoned by Al-Nusra, while other victims included prison guards and Al-Nusra fighters.
Among the dead were five civilians, including a child.
Russian warplanes have been conducting air strikes against the Islamic State organisation and "other terrorist groups" in Syria since September 30.
Although Al-Nusra and IS are both jihadist organisations, they are fierce rivals and regularly clash in Syria.
Al-Nusra also has tense relationships with non-jihadist rebel groups that oppose its extreme interpretation of Islamic law.
In Idlib, it has formed an alliance with rebel groups, including hardline faction Ahrar al-Sham.
The Army of Conquest coalition has expelled regime forces from Idlib province.
The Britain-based Observatory has an extensive network of sources inside Syria and identifies casualties by the type of aircraft flown and the munitions used.
Syria's conflict first erupted with anti-government demonstrations in March 2011 but quickly morphed into a war that has left more than 260,000 people dead.