The UN agency said the deaths in Tuesday night's strike had raised to almost 1,000 dead and 5,000 wounded the overall toll of an assault on Tripoli launched by military strongman Khalifa Haftar in April.

International Organization for Migration spokesman Joel Millman said six children and at least two migrants due to leave this week under a voluntary repatriation programme were among those killed.

Seventeen different nationalities, mainly African, were represented among the 600 migrants at the Tajoura detention centre, he told reporters in Geneva.

"According to IOM staff on site Thursday, 350 migrants, among them 20 women and four children, remain in detention there," said Millman.

Charlie Yaxlie, spokesman for the United Nations High Commissioner for Refugees, also speaking in Geneva, insisted on "a full and independent investigation to determine how this happened, who was responsible, and for those individuals to be held to account".

"The coordinates of detention centres in Tripoli are well-known to both sides in the conflict, and this was a preventable tragedy that never should have happened," Yaxlie said.