Hundreds of migrants returned to Libya this month are at high risk of human trafficking as worsening violence is allowing criminal gangs to thrive, according to the United Nations.

At least 953 migrants headed for Europe were picked up in the Mediterranean and sent back to Libya in the first two weeks of 2020, said the U.N.’s migration agency, the International Organization for Migration (IOM).

Mostly intercepted by Libya’s coast guard, backed by the European Union, they were all taken to detention centres where aid workers said they have first-hand testimony of migrants being sold to traffickers.

“There are threats to migrants of being trafficked and it’s definitely increasingly worrying,” IOM spokeswoman Safa Msehli told the Thomson Reuters Foundation.

“The security situation gives room to these criminal gangs and to these smuggling groups to prey on the migrants and their wish to leave a rather unsafe country,” she said.

Fighting has intensified in recent weeks between troops loyal to Libya’s internationally recognised government and the forces of eastern commander Khalifa Haftar, who are fighting for control of the capital, Tripoli.

The escalation has driven more migrants to try to escape the country, said IOM. In addition to the 953 brought back this year, 237 were also picked up by charity rescue ships and are awaiting a port that will take them.

During the same period last year, 23 bodies were recovered by the coast guard and no migrants were returned to Libya.

Libya has been a key gateway for Africans trying to reach Europe for years, and about 140,000 migrants and refugees were residing in Tripoli before the fighting broke out last April, according to the International Rescue Committee.

Even before that, migrants were preyed on by armed groups and traffickers, with slavery, rape and torture widely reported by human rights experts.

“There is no guarantee that prevents refugees and migrants returned to Libyan shores from falling back into the clutches of traffickers,” said Anais Deprade, a spokeswoman for the international medical charity Medecins Sans Frontieres (MSF).

“European governments can no longer pretend that Libya is a place of safety where refugees and migrants rescued at sea can be disembarked,” she said.

Deprade said MSF knew of people who had been sold into trafficking from some detention centres, and that migrants who escape the centres – which are notorious for abuse – were also vulnerable as they try to survive on their own in a war zone.