Blessing Obuson thought Russia's soccer World Cup would be an opportunity to find a job and flew into Moscow from Nigeria last June on a fan ID. Instead, she found herself forced to work as a prostitute.

Fan IDs allowed visa-free entry to World Cup supporters with match tickets, but did not confer the right to work. Despite that, Obuson, 19, said she had hoped to work as a shop assistant to provide for her 2-year-old daughter and younger siblings back in Nigeria's Edo state.

Instead, she said she was locked in a flat on the outskirts of Moscow and forced into sex work along with 11 other Nigerian women who were supervised by a madam, also from Nigeria.