Migrants from West Africa are being openly traded in "public slave markets" across Libya.

As a departure point for refugees trying to get to Europe, migrants arriving in Libya from sub-Saharan Africa are particularly vulnerable due to a lack of money and little in the way of documentation.

Survivors have told the International Organization for Migration (IOM) how there are slave markets and private prisons all over Libya.

Mohammed Abdiker, IOM's head of operation and emergencies, said: "The situation is dire. The more IOM engages inside Libya, the more we learn that it is a vale of tears for all too many migrants."

One survivor from Senegal spoke of how he was brought by smugglers across Niger in a bus to the southern Libyan city of Sabha, where he was due to risk a boat trip to Europe. When the middleman did not get his fee, the survivor was put up for sale along with other passengers.