At least 16 people died after a Friday night air strike on a prison in Syria's opposition-held Idlib province including both prisoners and staff, the Syrian Observatory for Human Rights, a Britain-based war monitor, said on Saturday.

Idlib is one of the most important strongholds of the opposition who seek to overthrow Bashar al-Assad, whose air force, along with that of his ally Russia, has been heavily bombarding opposition fighters there.

The Observatory said it had received information that some of those who died were shot dead while attempting to flee the prison after the air strike hit one side of it. Two jailers were among the 16 people killed.

The population of Idlib, located in northwest Syria, has been swelled by refugees including many of those who have left opposition-held enclaves elsewhere in the country after Assad forces and its allies forced them to surrender.

While parts of Idlib are controlled by Turkey-backed opposition fighters, including factions who fight under the banner of the Free Syrian Army, other areas are dominated by the Ahrar al-Sham and others by the Tahrir al-Sham alliance.

Tahrir al-Sham's strongest component is the former Nusra Front group which was al Qaeda's official branch in Syria until last year when it broke formal ties with the global movement.