Outrage and calls for an independent probe mounted on Wednesday as 44 migrants were killed in an air strike on a detention centre in Libya that the UN said could constitute a war crime.

UN chief Antonio Guterres denounced the "horrendous" attack and demanded an independent investigation, as a divided Security Council failed to condemn the strike.

Migrants carry the remains of their belongings from among rubble at a detention centre in Tripoli. Ismail Zitouny/Reuters

Libya's internationally recognised government and its arch-foe strongman Khalifa Haftar traded blame for the deadly assault, which the European Union called a "horrific" attack.

Bodies were strewn on the floor of a hangar in the Tripoli suburb of Tajoura, mixed with the blood-soaked clothes of migrants, an AFP photographer said.

"There were bodies, blood and pieces of flesh everywhere," a survivor, 26-year-old Al-Mahdi Hafyan from Morocco, told AFP from his hospital bed where he was being treated for a leg wound.

Hafyan said he had been detained in the centre for three months, after coming to Libya with a fellow Moroccan hoping to reach Europe across the Mediterranean.

His friend survived the attack unscathed, but his T-shirt was stained with other people's blood. "We were lucky. We were at the back of the hanger."

Tuesday night's strike left a hole around three metres (10 feet) in diameter in the hangar, surrounded by debris ripped from the metal structure by the force of the blast.

At least 44 people were killed and more than 130 severely wounded, the UN said.

The UN shared the coordinates of the Tajoura centre east of Tripoli with the warring sides to ensure that civilians sheltering there were safe, Guterres said.

UN envoy to Libya Ghassan Salame said the "attack clearly could constitute a war crime, as it killed by surprise innocent people whose dire conditions forced them to be in that shelter."

Agence France-Presse