Guards at the Libyan migrant centre that was hit by a deadly air strike on Wednesday may have shot at refugees who were trying to flee the scene, the UN said.
The Tajoura detention centre for migrants on the edge of the capital Tripoli was hit in the early hours of the morning, killing at least 53 people, including six children, and wounding many more.
UN Secretary General Antonio Guterres on Wednesday denounced the "horrendous" attack and demanded an independent inquiry.
The UN-backed Government of National Accord blamed Field Marshal Khalifa Haftar's Libyan National Army for the strike, while the LNA blamed militia aligned with the government.
A report from the UN Office for the Co-ordination of Humanitarian Affairs said the incident involved two air strikes; one hitting an unoccupied garage and the other a hangar housing about 120 refugees and migrants.
"There are reports that following the first impact, some refugees and migrants were fired on by guards as they tried to escape," the UN agency's report said.
It also suggested that the death toll could rise because bodies were still being recovered from the rubble.
A spokesman for Mr Guterres said the exact co-ordinates of the centre had been provided to the warring parties to ensure it was avoided.
The death toll is already the highest publicly reported from an air strike or shelling since Field Marshal Haftar launched an offensive three months ago to take Tripoli, the base of Libya's internationally recognised government.
"The number of civilian casualties caused by the conflict has almost doubled as a result of this single attack," the UN report said.
Ghassan Salame, the Secretary General’s special envoy to Libya, said the attack could constitute a war crime and urged the international community to punish those who ordered, carried out and provided arms for the strike.
There are still more than 500 people at the detention centre at Tajoura.
Libya is one of the main departure points for African migrants fleeing poverty and war to reach Italy by boat, but many are intercepted at sea and taken back by the Libyan coastguard.
