Of the 600 migrants that were in the centre, "300 were still there" Thursday and receiving humanitarian assistance from the IOM, Safa Msehli, the communications director for the organisation in Libya, told AFP.

Msehli was unable to confirm reports that dozens of migrants had fled on Tuesday night after the raid in the Tripoli suburb of Tajoura which also left 130 wounded.

But the IOM said in a statement that its teams had "located" and transferred to hospital "a group of injured migrants who left Tajoura after the attack in the surrounding neighbourhood".

"Innocent lives were lost in the attack on Tuesday night, and immediate action is needed from all sides," the IOM's Libya chief of mission, Othman Belbeisi, said in the statement.

"The suffering of migrants in Libya has become intolerable. It must be clear to all that Libya is not a safe port and that thousands of lives remain at imminent risk," he added.

The tragedy provoked an international outcry, but the divided UN Security Council failed to unanimously condemn the attack in an emergency meeting on Wednesday after the United States did not endorse a proposed statement.

Libya's internationally recognised government and its arch-foe strongman Khalifa Haftar traded blame for the deadly assault.

According to the IOM, of the more than 600 migrants detained in Tajoura, 187 were registered with its "Humanitarian Voluntary Return" programme which helps migrants go back to their home countries.

The IOM "continues to call for an end to the arbitrary detention and reminds all parties that civilians are not a target," the IOM statement added, reiterating that some 3,300 migrants are still detained in and around the Libyan capital in centres "considered at-risk".

UN agencies and humanitarian organisations repeat regularly their opposition to the return of migrants arrested at sea to the North African country that has been wracked by chaos since the 2011 uprising against dictator Moamer Kadhafi.

Rights groups say migrants face horrifying abuses in Libya, which remains prey to a multitude of militias vying for control of the oil-rich country.

Their situation has worsened since Haftar launched on April 4 an offensive to conquer Tripoli, the seat of the internationally recognised Government of National Accord (GNA).