Up to 40,000 Nigerian girls are being forced to work as prostitutes in Mali "slave camps", Nigerian officials say.

The girls have often been promised jobs in Europe but ended up in brothels in the capital or mining towns, said the government's anti-trafficking agency.

The brothels are run by older Nigerian women who prevent them from leaving and take all their earnings. The agency said it was working with Malian police to free the girls and help them return to Nigeria. Nigeria's National Agency for the Prohibition of Traffic in Persons (Naptip) said officials visited Mali this month to follow up "horrendous reports" from victims, aid workers and clergy in Mali. They found hundreds of brothels, each housing up to 200 girls, run by Nigerian "madams" who force them to work against their will and take their earnings. Naptip estimates that there are between 20,000 and 40,000 Nigerian women and girls living in such conditions.