GENEVA, April 11 (Reuters) - Growing numbers of African migrants passing through Libya are traded in what they call slave markets before being held for ransom, forced labour or sexual exploitation, the U.N. migration agency said on Tuesday.
West African migrants interviewed by the International Organization for Migration (IOM) have recounted being bought and sold in garages and car parks in the southern city of Sabha, one of Libya's main migrant smuggling hubs.
Migrants are traded for between $200 and $500 and are held on average for two or three months, Othman Belbeisi, head of the IOM's Libya mission, told journalists in Geneva.
"Migrants are being sold in the market as a commodity," he said. "Selling human beings is becoming a trend among smugglers as the smuggling networks in Libya are becoming stronger and stronger."
The migrants - many from Nigeria, Senegal and Gambia - are captured as they head north towards Libya's Mediterranean coast, where some try to catch boats for Italy.
Along the way they are prey to an array of armed groups and smuggling networks that often try to extort extra money in exchange for allowing migrants to continue.
Most migrants are used as day labourers in construction or agriculture. Some are paid and others are forced to work without pay.
"About women, we heard a lot about bad treatment, rape and being forced into prostitution," Belbeisi said.
The IOM said it had spoken to one Senegalese migrant who was held in a Libyan's private house in Sabha with about 100 others, who were beaten as they called their families to ask for money for their captors. He was then bought by another Libyan, who set a new price for his release.
Some migrants who cannot pay their captors are reportedly killed or left to starve to death, and when migrants die or are released, others are purchased to replace them, the IOM said.
Migrants are buried without being identified, with families back home uncertain of their fate.
"What we know is that migrants who fall into the hands of smugglers face systematic malnutrition, sexual abuse and even murder," Mohammed Abdiker, IOM's Director of Operations and Emergencies, said in a statement. "We are hearing about mass graves in the desert."
Libya is the main gateway for migrants attempting to reach Europe by sea, with more than 150,000 people making the crossing in each of the past three years.
So far this year an estimated 26,886 migrants have crossed to Italy, over 7,000 more than during the same period in 2016. More than 600 are known to have died at sea, while an unknown number perish during their journey north through the desert.
(Reporting by Stephanie Nebehay; Writing by Aidan Lewis; Editing by Tom Heneghan)