At least 44 people were killed and more than 130 wounded in an airstrike on a migrant detention center in the Libyan capital Tripoli, the U.N. mission to the country said on Wednesday.

U.N. Libya envoy Ghassan Salame condemned the attack, saying in a statement that the strike "clearly amounts to the level of a war crime."

It was the highest publicly reported toll from an air strike or shelling since eastern forces loyal to Khalifa Haftar launched an offensive three months ago with ground troops and aircraft to take the capital, base of Libya's internationally recognized government.

Chairperson of the African Union (AU) Commission Moussa Faki Mahamat, in a statement, condemned the strike and called for "an independent investigation to be conducted to ensure that those responsible for this horrific crime of innocent civilians, be brought to account."

"The chairperson reiterates his call for an immediate ceasefire, and for all parties to ensure the protection and safety of all civilians, especially the migrants trapped in detention centers," the AU statement added, calling on the international community to "redouble efforts" to bring the warring parties to the negotiation table.

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.

No one has so far claimed responsibility but pro-Haftar media reported Tuesday night a "series of air raids" in Tripoli and Tajoura.

UHHCR, the UN refugee agency, 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."

(With input from Reuters and AFP)