The United Nations on Thursday raised the death toll of Tuesday's airstrike on a migrant detention center in the suburb of Libyan capital Tripoli to 53, including six children. Another 130 were injured.

A UN humanitarian report said there were two airstrikes, one hitting an unoccupied garage and another hitting a hangar containing around 120 refugees and migrants.

Nigerian President Muhammadu Buhari on Wednesday strongly condemned the deadly airstrike, describing it as "inhuman, wicked, and callous on helpless and defenseless people." Some Nigerians were among the 40 people killed in that airstrike, the International Organization for Migration (IOM) said on Wednesday, without disclosing the exact number.

UN Secretary-General Antonio Guterres on Wednesday voiced outrage and called for an independent investigation.

Libya is a main departure point for undocumented migrants from African and Arab countries trying to reach Europe by sea, but many of them were stopped by the Libyan coast guards funded by the European Union and detained in detention centers in Libya. 

(With input from agencies)