Russian strikes today on a prison complex run by Al-Qaeda's Syrian affiliate killed at least 57 people and wounded 30 others, many critically, a monitor said, giving a revised toll.

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.

A statement said that among the militants killed were 23 Nusra fighters.

"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.

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.

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.