Satellite imagery of the aftermath of the airstrike that hit a migrant centre in the Tajoura suburb of Tripoli, Libya
* More than 130 wounded in strike in Tripoli suburb
* Migrant centre located next to military camp
* UN had called for centre to be moved after strike in May
* Haftar's eastern forces stalled in bid to seize 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 more than 130, the United Nations 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".
"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.
UN human rights chief Michelle Bachelet said it was the second strike on the centre during the recent fighting, even though its coordinates had been communicated to the warring sides. It may - depending on the precise circumstances - be a war crime, she added.
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 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.
Frightened migrants were still at the detention centre after the strike, which partially destroyed the hangar. "Some people were wounded, and they died on the road, on their way running, and some people are still under the debris so we don't know what to say," said Othman Musa, a migrant from Nigeria.
"All we know is we want the UN to help people out of this place because this place is dangerous," he said.
Clothes, flip-flops, bags and mattresses were littered on the floor next to what remained of limbs of dead. Blood stains coated some walls.
"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's Libyan National Army (LNA) 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.
The UN called for an independent investigation and for perpetrators to be held to account.
In a statement, the Tripoli-based government blamed the "war criminal Khalifa Haftar" for the incident.
Moussa Faki Mahamat, chairman of the African Union Commission, demanded an immediate ceasefire and an independent investigation "to ensure that those responsible for this horrific crime of innocent civilians be brought to account".
An LNA official denied that his force had hit the detention centre, saying that militias allied to Tripoli had shelled it after a precision air strike by the LNA on a military camp.
The LNA air campaign has failed to take Tripoli in three months of fighting, and last week lost its main forward base in Gharyan to Tripoli's forces.
Both sides enjoy military support from regional powers. The LNA has been supplied for years by the United Arab Emirates and Egypt.
The conflict threatens to disrupt oil supplies, boost migration across the Mediterranean to Europe, scupper UN plans for an election to resolve the rivalry between the parallel administrations in east and west - and create a security void that Islamist militants could fill.