The UN says it has received reports that guards fired on migrants who tried to flee air strikes on a detention centre near Libya's capital Tripoli on Tuesday.
The UN says it believes at least 53 migrants died and 130 were hurt in the air strikes on the Tajoura centre.
UN High Commissioner for Human Rights Michelle Bachelet has said the air strikes could constitute a war crime.
The government and an opposing militia have blamed each other for the attack.
Thousands of migrants trying to reach Europe, many of them sub-Saharan Africans, are being held in Libyan detention camps. Dozens who set off from Libya were feared dead as a boat capsized off Tunisia on Wednesday.