At least 20,000 missing women and girls were found to have been trafficked from Nigeria to Mali, the head of Nigeria’s anti-trafficking agency said on Tuesday, after they received information revealing that the young women were living in “slave-like” conditions, according to CNN .

“The conditions are horrible,” said Julie Okah-Donli, the director-general of the National Agency for the Prohibition of Trafficking in Persons (NAPTIP). “They are kept in shanties in the thick of the forest where they cannot escape and with the ‘madames’ watching over them."

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Some of the girls were abducted on their way to school. But most were tricked by human traffickers who promised them jobs in Malaysia. Instead, the girls, some as young as 16, were left stranded in Mali, where many were sold into prostitution, CNN reports.

Each year, thousands of Nigerians are illegally trafficked around the world. Nigerians have been trafficked to at least 40 countries, where most survivors face abuse and exploitation, according to a report from the US Department of State. About 97% of these human trafficking survivors are women, and 77% have been sexually abused or exploited by their traffickers.

For NAPTIP, rescuing survivors is an ongoing effort. Its rescue mission Operation Timbuktu helped 104 trafficking survivors escape from three brothels in Bamako which is Mali’s capital in 2011. And the organization helped repatriate dozens of the more recently trafficked women late last year, Reuters reports.

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Officials hope to continue helping survivors escape prostitution and return back to Nigeria. Okah-Donli said that her agency NAPTIP is teaming up with the International Organization for Migration, a UN-affiliate, to help survivors escape sex trafficking.

While they don’t have an estimated number of Nigerian girls who were trafficked to other parts of Africa, the agency plans to send help to neighboring countries. Nigerians are predominantly trafficked to Europe, but Ghana, Burkina Faso, and the Ivory Coast are common places for trafficking to occur as well.