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.
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.
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