Libya's U.N.-supported government has asked the Security Council to appoint a "fact-finding mission" to investigate alleged violations by the forces attacking the capital, according to a letter circulated Thursday.

Ambassador Elmahdi Elmajerbi said in an April 18 letter to the council president that the violations include "the killing and displacement of civilian citizens, the destruction of public and private properties, the recruitment of children and involving them in wars and hostilities."

It also cited "targeting the civilian neighborhoods and populated areas and exposing them to heavy artillery and Grad missiles."

Forces loyal to Field Marshal Khalifa Hifter launched an offensive in Tripoli earlier this month, and are battling militias loosely allied with the U.N.-supported government.

The U.N. health agency says the fighting has killed more than 270 people, including civilians, and wounded nearly 1,300.

U.N. spokesman Stephane Dujarric said at a briefing in New York on Thursday that the U.N. refugee agency and the International Organization for Migration transferred 328 refugees and migrants from the Qasr Bin Ghashir detention center to a safer place on Wednesday and the remaining refugees and migrants were being transferred Thursday.

"The relocation follows violence at the center on Tuesday, and was carried out with support from Libyan authorities and the U.N.," Dujarric said. "A humanitarian pause was coordinated and facilitated with all parties to allow for the safe passage of the migrants and refugees."