UN Secretary-General Antonio Guterres says he is horrified at reports of Black African immigrants being sold as slaves in Libya.
His comments followed the broadcast by CNN of footage of what it said was an auction of men offered to Libyan buyers as farmhands and sold for $400.
Guterres said he had asked for an investigation. Libya has become the focus of European nations seeking to restrict the numbers of migrants from the Middle East and Africa crossing the Mediterranean.
Migrants are held in detention centers in the north African county, which Human rights groups have compared to concentration camps.