Nearly 40 illegal migrants, mostly from Africa, were killed on Tuesday in an air strike on their detention center in the Libyan capital, Tripoli.
The Libyan National Army (LNA) denied that it had targeted the center, saying that armed factions allied to the Tripoli-based Government of National Accord were responsible for the attack.
It said that the factions carried out the attack after the LNA struck a military camp.
At least 70 were also wounded in the raid on Tajoura, an emergency services spokesman told AFP.
The suburb of Tajoura has several military sites belonging to pro-GNA armed groups.
The UN refugee agency said it was "extremely concerned" at reports of the strikes on the migrant center.
"UNHCR is extremely concerned about news of air strikes targeting Tajoura detention center east of Tripoli, and accounts of refugees and migrants deceased," it tweeted.
"Civilians should never be a target."
Despite chronic instability, Libya remains an important transit point for migrants fleeing conflict and instability in other parts of Africa and the Middle East.
