The UN says information has emerged that Libyan guards fired on refugees trying to flee air strikes on a migrant detention centre.
The UN says it has information that Libyan guards shot at refugees and migrants trying to flee from air strikes that killed at least 53 people, including six children, in a migrant detention centre.
A report from the UN Office for Coordination of Humanitarian Affairs (OCHA) says there were two air strikes late on Tuesday, one hitting an unoccupied garage and one hitting a hangar containing around 120 refugees and migrants.
"There are reports that following the first impact, some refugees and migrants were fired upon by guards as they tried to escape," the OCHA report said.
The Tripoli-based interior ministry denied this categorically in a statement.
Bodies were still being recovered from the rubble, the UN report said, suggesting the death toll could rise.
There are still about 500 people at the detention centre at Tajoura, east of Tripoli.
Four Nigerians are set for release to the Nigerian embassy on Thursday and a plan is in place for 31 women and children to be sent to the UN refugee agency's departures facility in Tripoli.
Interior Minister Fathi Ali Bashagha said the Tripoli government was considering closing all detention centres for migrants and releasing the inmates.
Thousands are held in centres in Tripoli alone.
It is the highest death toll from an airstrike 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 capital.
Journalist Sally Hayden shared text messages she received from the detainees during the attack.
“Airstrike bombing [is] closer than last time to our cell,” one wrote in a text.
From his hospital bed in Tripoli, wounded Al-Mahdi Hafyan recalls the nighttime "massacre" when the airstrike hit.
Surviving the assault with a leg injury, the 26-year-old Moroccan remembered seeing "bodies, blood and pieces of flesh everywhere."
Hafyan had been detained in the centre for three months, after coming to Libya with a fellow Moroccan hoping to reach Europe by crossing the Mediterranean.
His friend survived the attack unscathed, but his t-shirt was stained with other people's blood.
"We were lucky. We were at the back of the hangar," he told AFP.
Both men feared being held in another detention facility in Libya, where rights groups say migrants face horrifying abuses.
"We want to get out of here, if not we will be imprisoned again. We want to go home," said Hafyan, pushing to leave the hospital.
United Nations Libya envoy Ghassan Salame condemned Tuesday's 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.
The Libyan capital's only functioning airport suspended flights on Wednesday after an air raid claimed by strongman Khalifa Haftar's forces, airport authorities said in a statement.
The attack did not cause casualties or damage, a security source at Mitiga airport said.
But Ahmad al-Mesmari, a spokesman for Haftar's self-styled Libyan National Army, said that a "command centre for drones at Mitiga" was destroyed in the raid.
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.
"The number of civilian casualties caused by the conflict has almost doubled as the result of this single attack," the OCHA report said, a day after UN officials said the air strikes may have constituted a war crime.
The United Nations has repeatedly said Libya is not a safe place for rescued migrants to be sent back to and has called for refugees and migrants to be released and given safe shelter.
It has declined to directly criticise Italy for shutting its doors but said European states must help to resolve the conflict in Libya in order to stop people taking perilous sea journeys.
The centre is close to a military base and an air strike on nearby Libyan government forces on 7 May injured two people in the centre.
Despite the risks, Libyan authorities had continued to transfer migrants and refugees into the centre, according to the UN report.
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.
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.
"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), 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 refugee agency and International Organisation for Migration called for an independent investigation and bringing perpetrators to account.
"Coordinates of such centres in Tripoli are well known to combatants, who also know (that) those detained at Tajoura are civilians," the two UN agencies said in a joint statement.
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.