The Syrian Observatory for Human Rights said the air raids on an Al-Nusra Front building prison near a popular market in northwestern Idlib province killed 21 civilians, 29 militants and seven detainees.
The building in Maarat al-Numan housed the group's religious court and a jail.
The Britain-based Observatory, which relies on a large network of sources inside Syria, said a child and two women were among the civilians killed in the strikes.
"The toll from the Russian raids on the Al-Qaeda-run prison has risen to at least 57 killed and 30 wounded, many in critical condition," said the Observatory which had earlier given a toll of 39 dead.
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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.
The Army of Conquest coalition has expelled regime forces from Idlib province.
Syria's conflict first erupted with anti-government demonstrations in March 2011 but expanded into a war that has left more than 260,000 people dead.