The "overwhelming majority" of women and older girls who passed through Libya as migrants reported being gang-raped by traffickers or having witnessed others taken away to be sexually abused, the United Nations said Thursday in a report based on hundreds of interviews.

The Office of the High Commissioner for Human Rights said its report , authored along with the U.N. Support Mission in Libya, turned up "unimaginable horrors" among migrants who seek to reach Europe through the largely lawless country. The report covers January 2017 to August 2018.

The U.N. said investigators pulled together 1,300 first-hand accounts detailing "a terrible litany of violations and abuses committed by a range of state officials, armed groups, smugglers and traffickers against migrants and refugees." Those included unlawful killings, torture, arbitrary detention, gang rape, slavery, forced labor and extortion.

The report comes as European Union leaders have pursued efforts to beef up the bloc's external borders to stop large numbers of migrants from entering Europe. Thousands of migrants who have been caught trying to cross the Mediterranean from Libya have been returned to the North African country and put in detention centers, often with squalid conditions.

The accounts emerged during regular visits to 11 detention centers in Libya, and in interviews with migrants in Italy — a EU country that some migrants reached — and Nigeria, where others were repatriated.

"The overwhelming majority of women and older teenage girls interviewed by UNSMIL reported being gang raped by smugglers or traffickers or witnessing others being taken out of collective accommodations to be abused," the report said. Spokeswoman Ravina Shamdasani confirmed that the abuse was sexual.

The report builds upon recent concerns expressed by U.N. officials and advocacy groups. The head of the U.N. refugee agency, Filippo Grandi, said in October that Libya — the main launch point today for migrants hoping to cross the Mediterranean Sea — has strengthened its coast guard in recent months, but not its other institutions.

Alluding to a similar joint study published two years ago, the report said Libyan authorities "have thus far appeared largely unable or unwilling to put an end to violations and abuses committed against migrants and refugees."

It also offered recommendations.

"Libya's approach to managing migration must be overhauled, with human rights protections placed at the center of response plans" the report said. It called for the decriminalization of irregular entry into the country and the release of all migrants detained "arbitrarily."

Until then, it said, women should be separated from male detainees, "guarded only by adequately trained female officers."

As for the EU, the report said the bloc should set up "independent monitoring mechanisms" to ensure human rights protections along with its "donor agreements" with Libyan authorities and others. The EU should also "refrain from encouraging a shifting responsibility for search and rescue operations in international waters" to the Libyan coast guard, it said.

Countries of origin should improve regional cooperation and better inform migrants of the risks they run, the report said.

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This story has been corrected to say the majority either were gang-raped or witnessed others being taken to be abused, not that all were gang-raped themselves, which was based on an incorrect U.N. press release.