An attack which killed more than 53 migrants at a detention centre outside the Libyan capital could constitute a war crime, a UN official said.
At least 130 people were injured in the attack, which the Libyan government blamed on an air strike by forces loyal to a warlord, General Khalifa Haftar.
Gen Haftar's forces accuse the government side of shelling the centre.
Most of the dead are believed to be sub-Saharan Africans who were attempting to reach Europe from Libya.
Thousands of migrants are being held in government-run detention centres in Libya. The location of the centre attacked on Tuesday and the information that it housed civilians had been passed to all parties in Libya's conflict, the UN's High Commissioner for Human Rights, Michelle Bachelet, said.