Read the full story from BBC News HERE.
Nigerian girls are being forced to work as prostitutes in Mali "slave camps", say officials in Nigeria.
The girls, many of them under age, have often been promised jobs in Europe but ended up in brothels, 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.
There has been no official comment from the Mali authorities.
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 said there were 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.
"We are talking of thousands and thousands of girls," Simon Egede, Executive Secretary of Naptip, told a news conference in Abuja.
"We are talking of certainly between 20,000 and about 40,000," he said, but did not give details of how the figure had been reached.
In a statement, Mr Egede said girls were "held in bondage for the purposes of forced sexual exploitation and servitude or slavery-like practices".
"The madams control their freedom of movement, where they work, when they work and what they receive," he said.The trade is centred around the capital Bamako and large cities, but the most notorious brothels are in the mining towns of Kayes and Mopti, where the sex workers live in "near slavery condition", said Naptip.
Many of the brothels there also had abortion clinics where foetuses were removed by traditional healers for use in rituals, said Mr Egede.