African refugees attempting to migrate through Libya to Europe are being captured and sold in public slave markets, the International Organization for Migration (IOM) reports.

Word of the slave markets reached the organization after survivors were able to pay their ransom or escape by other means.

Separate from the migrant crisis engendered by the Syrian Civil War, massive numbers of Africans from sub-Saharan Africa are risking their lives to migrate through Northern Africa and on into Europe. Libya, for years a lawless region with many areas governed only by militia groups, has become the main hub for migrants desperate to reach Europe.

One migrant, a Senegalese man who was held for months, described the conditions he and hundreds of others endured once they were captured by armed groups as they made the trek into Libya. Typically, once they become prisoners, they are forced to call their families and demand ransoms for their release. While they speak to their families over the phone, their captors beat them, so that their families can hear them endure the torture.

One man was released only after his father spent nine months gathering the necessary funds, which required him to sell the family’s home.

Other migrants who have certain trade skills like carpentry are sold to local groups who then put them to work. Women migrants are reportedly being sold and used as sex slaves by some Libyans.

Economic and demographic conditions in African countries, in particular Niger, Nigeria, Senegal, and the Gambia, are driving the migrant boom. In the last few years, over 150,000 African migrants have passed through Libya onto Europe by sea. African leaders are desperate for assistance, with Niger’s president suggesting to German Chancellor Angela Merkel that something akin to a “Marshall Plan”—the American-led initiative to economically restore a shattered Germany after World War Two—would be needed to stem the tide of migrants fleeing his country.

With soaring birth rates and countries unable to support and educate their citizens, hundreds of thousands of Africans are subjecting themselves to smugglers who claim they will find them passage to Europe.