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

Idlib is one of the most important strongholds of rebels, including jihadist factions, who seek to overthrow President Bashar al-Assad, whose air force, along with that of his ally Russia, has been heavily bombarding insurgents 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 rebel-held enclaves elsewhere in the country after the army and its allies forced them to surrender.

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