An air strike hit a detention centre for mainly African migrants in a suburb of the Libyan capital Tripoli late on Tuesday, killing at least 44 people and wounding more than 130, the UN mission to Libya said.

It was the highest publicly reported toll from an air strike or shelling since eastern forces under Khalifa Haftar launched a ground and aerial offensive three months ago to take Tripoli, the base of Libya's internationally recognised government.

United Nations Libya envoy Ghassan Salame condemned the strike, saying it "clearly amounts to the level of a war crime".

A migrant picks up her belongings from among rubble at a detention centre near Tripoli. Ismail Zitouny/Reuters

"The absurdity of this ongoing war has today reached its most heinous form and tragic outcome with this bloody, unjust slaughter," Salame said in a statement.

Libya is one of the main departure points for African migrants, fleeing poverty and war, to try to reach Italy by boat, but many are picked up and brought back by the Libyan coast guard, supported by the European Union.

Thousands are held in government-run detention centres in what human rights groups and the United Nations say are often inhuman conditions.

The UNHCR refugee agency had already called in May for the Tajoura centre, which holds 600 people, to be evacuated after a projectile landed less than 100 metres (330 feet) away, injuring two migrants.

Tajoura, east of Tripoli's centre, is home to several camps belonging to forces allied to the internationally recognised government, which have been targeted by air strikes for weeks.

Photos published on Tuesday showed African migrants undergoing surgery in a hospital after the air strike. Others lay on beds, some covered in dust or with bandaged limbs.

"Our teams had visited the centre just yesterday (Tuesday) and saw 126 people in the cell that was hit. Those that survived are in absolute fear for their lives," medical charity Medecins Sans Frontieres (Doctors Without Borders) said in a statement.

Haftar Assault On Tripoli

Haftar's Libyan National Army (LNA), allied to a parallel government based in eastern Libya, has seen its advance on Tripoli held up by robust defences on the outskirts of the capital, and said it would start heavy air strikes after "traditional means" of war had been exhausted.

His attempt to capture Tripoli has derailed UN attempts to broker an end to the chaos that has prevailed in the oil- and gas-producing North African country since the violent, NATO-backed overthrow of Muammar Gaddafi in 2011.

Reuters