TRIPOLI • 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 over 130, the United Nations said.

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

UN Libya envoy Ghassan Salame condemned the strike, saying it "clearly amounts to the level of a war crime". He said in a statement: "The absurdity of this ongoing war has today reached its most heinous form and tragic outcome with this bloody, unjust slaughter."

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 coastguard, supported by the European Union.

Thousands are held in government-run detention centres in what human rights groups and the UN 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 100m away, injuring two migrants.

The hangar-type detention centre is next to a military camp, one of several in Tajoura, east of Tripoli's centre, which have been targeted by air strikes for weeks.

General Haftar's Libyan National Army force, 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.