At least 16 people have been killed overnight in air strikes on a prison in Syria's rebel-held Idlib Province, the Syrian Observatory for Human Rights monitoring group says.

The observatory said on March 25 the dead included both prisoners and staff.

Syrian government forces have been heavily bombarding insurgents in Idlib, which is one of the most important strongholds of rebels seeking to overthrow President Bashar al-Assad.

Parts of the northwestern province are controlled by Turkey-backed rebels, including the Free Syrian Army, while other areas are dominated by hard-line groups.

The provincial capital of the same name is mostly held by rebels led by the Al-Qaeda-linked Jabhat Fatah al-Sham.

Idlib has regularly been targeted by both Russian and government air strikes as well as raids launched by the U.S.-led coalition fighting the Islamic State extremist group.

Russia, a major ally of the Syrian government, has carried out an air campaign in Syria since September 2015.

More than 320,000 people have been killed in Syria since the conflict began six years ago with antigovernment protests.

Based on reporting by AFP, Reuters, and dpa