Two years ago a woman came to Blessing's hometown in the south of Nigeria and told the young woman there was work for her in Russia. She told Blessing she would have a job in a supermarket, and that it would take the her just five or six months to earn the money to reimburse the costs of the visa and the journey to Russia. After paying back the $40,000, Blessing could keep all the money she made, the woman said.

Blessing said her parents are very poor and that the idea of going to Russia to help them and to save money for her education appealed to her. She agreed.

Only when she arrived, did she discover everything the woman had said was a lie.

There was no supermarket job. Instead, Blessing told DW, she was forced to work as a prostitute.

She was driven around Moscow to have sex with men. One night, she was taken to an apartment building where one man was apparently waiting for her. When she got inside, she discovered there were eight men. She was forced to sleep with all of them, she said. When she refused to have sex without a condom, they took back the money they had paid and beat and molested her, she said.

Then they threw her from the fourth floor of the building.

Blessing broke her hip when she hit the ground. She spent two-days on life-support in the hospital until her treatment was stopped because, she said, she could not afford to pay. She now cannot walk properly and is confined to a wheelchair.

Trafficked on student visas

Blessing's story is not uncommon, said Kenny Kehinde, who works with several Moscow NGOs focused on preventing human trafficking. Around 2,000-3,000 Nigerian girls - many from poor, remote villages - are brought to Russia every year for sex work, he said.

"This is international modern-day slavery, where the girls are brought here with the help of some Russian government officials, some Nigerian authorities and so-called 'madams' [pimps] who exploit these girls for sex in Russia," said Kehinde.

Kehinde said many girls forced into sex work in Russia came to the country on student visas

Most of the girls Kehinde dealt with had come to Russia on student visas, he said.

Such visas are not easy to obtain as universities must provide supporting material for the applications.

Usman Gafai, head of mission at the Nigerian Embassy in Moscow, said he, too, was aware of Nigerians being trafficked for sex to Russia.

"Ten years ago, it was not such a huge problem as this," he told DW. "Those involved are an international cartel. On a daily basis they are growing and making money out of it."

The Russian government needed to "carry out proper scrutiny of visa applicants back in Nigeria," Gafai said. "The majority come to Russia on a student visa, and I want to see more scrutiny of that."

Kehinde said illiterate teenagers were being trafficked.

"How can you bring a girl of 14- or 15-year old to study in a university, when she cannot even read and write?" he asked.

Migration violations

DW was able to examine passports and migration documents belonging to six Nigerian girls, including Blessing, that showed they had arrived in Russia on student visas.

The Smolny Institute of the Russian Academy of Education in Saint Petersburg told DW it had issued visa support documents in 2014 for Blessing to study a Russian-language course in preparation for entering university. However, in an emailed statement to DW, the university's rector, Gaidar Imanov, said she never arrived at the institute, and the university had no knowledge of whether she had entered the country.

Similarly, the Baltic Humanitarian Institute, another St. Petersburg university, confirmed via email it had issued documents to a would-be student from Nigeria who had never made contact to begin her course in Russia.

Both universities rejected the notion that their staff may have been paid to provide documents to students who were not genuine or to traffic girls to Russia for sex, calling the allegation "fiction" and "absolutely baseless."

DW examined numerous documents belonging to girls who were trafficked to Russia and exploited

The Russian Ministry for Foreign Affairs, whose embassies issue visas, told DW in a written statement that all students in Nigeria undergo an interview "without fail." It also said border officials do not allow entry into the country without confirmation from the university where they are due to study. The ministry said it "wanted to emphasize" that individuals are "personally responsible for adhering to migration legislation."

Russia's Federal Migration Service told DW that it "regularly checks" for migration violations and that immigration law had been broken in more than 325,000 cases in the first two months of 2016. Despite several requests, the agency did not explain how Blessing - and girls like her - could have entered Russia on student visas and apparently disappeared from authorities' sight for years into the sex trade.

Tackling trafficking

Despite legislation meant to prevent human trafficking, Russia has not shown a full commitment to tackling the problem, said Andrew Bogrand of the NGO Democracy International.

"Prosecution, although existent, is very limited," he said. "More alarming, according to Russia's few women's rights NGOs, is the almost complete lack of shelter space for women who are victims of sex trafficking or domestic violence.

"Corruption and trafficking are inextricably linked - and Russia fares poorly in most corruption indexes," he continued. "As long as the state continues to turn a blind eye to the problem of corruption, trafficking will flourish."

'Stay home'

Blessing recently returned to Nigeria and hopes to resume her studies. But her time in Russia has changed her life forever. It remains unclear whether she will be able to walk properly again.

She has a message for other Nigerian girls who are offered jobs abroad: "Stay back home, learn to work. Even though the pay is small, it is much better than coming here to suffer or lose your life."