Nearly 40 migrants were killed in an air strike Tuesday night on their detention center in a Tripoli suburb blamed on Libyan General Khalifa Haftar, who has been trying for three months to seize the capital.
At least 70 were also wounded in the raid on Tajoura, an emergency services spokesman told AFP.
"This is a preliminary assessment and the toll could rise," said spokesman Osama Ali.
He said 120 migrants were detained in the hangar which was directly hit by the strike.
Bodies were strewn on the floor of the hangar, mixed with the belongings and blood-soaked clothes of migrants, an AFP photographer said.
Rescuers were searching for survivors under the rubble, while dozens of ambulances rushed to the scene.
In a statement, the internationally recognized national unity government (GNA) based in Tripoli denounced the attack as a "heinous crime" and blamed it on the "war criminal Khalifar Haftar."
Haftar, who controls much of eastern and southern Libya, in early April launched an offensive to take the capital.
The GNA accused pro-Haftar forces of having carried out a "premeditated" and "precise" attack on the migrant center.
No one has so far claimed responsibility but pro-Haftar media reported Tuesday night a "series of air raids" in Tripoli and Tajoura.
The suburb of Tajoura, which has several military sites belonging to pro-GNA armed groups, is regularly targeted in air raids by Khalifa's forces.
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."
Libya, wracked by chaos since the 2011 uprising against dictator Moamer Kadhafi, has become a major conduit for migrants seeking to reach Europe.
Rights groups say migrants face horrifying abuses in the North African country, which remains prey to a multitude of militias vying for control of the oil-rich country.
