GENEVA (Reuters) - Thousands of Nigerian migrant girls being brought to Italy are in great danger of being forced into prostitution in Europe, often naive about their fate at the hands of traffickers, the International Organisation for Migration (IOM) said on Friday.

The number of female Nigerians, many of them minors, arriving by sea in Italy surged over the past three years, from 1,454 in 2014 to 11,000 last year. Some 4,000 have arrived so far in 2017, the United Nations agency said.

Based on data collected at landing sites, it estimates that 80 percent are “potential victims” of trafficking as sex workers, noting that the girls are often told the trip is free.

“These girls, who are increasingly younger ever year, are brought to Italy and to Europe for sexual exploitation,” IOM spokesman Flavio Di Giacomo told a Geneva briefing by telephone from Rome.

“They are taken by traffickers and then they are forced to be prostitutes in the streets in Italy. And sometimes, as far as we know these girls are also sent to other European countries such as Spain, Germany, France, Austria,” he said.

Many of the Nigerian girls are from poor families in Edo state who are “psychologically manipulated” during a voodoo ritual before leaving, he said. They are accompanied by a madame linked to the traffickers on the long journey through Niger and Libya.

“The increasing number of minors arriving really do not have any idea even what prostitution is, what sex is,” Di Giacomo said.

“But many of them understand the real purpose of this journey ... because many times they are forced to prostitute themselves in Niger, in Libya especially, in brothels there,” he said. “So they understand they are not going to work as hairdressers.”

IOM officials try to intercept the Nigerian girls at landing points and inform them about the prostitution ring and their right to protection, Di Giacomo said.

The agency, which helps those who decide to flee the networks, reported the cases of 425 girls last year to police.

“A few days ago we had the story of this young girl, let’s call her Precious, a 17-year-old girl. She has been told she was going to work at a hair salon, she didn’t have any idea she was going to be forced to prostitute herself.

“It was only when she arrived she found out that it was not real. The job was a lie and she was forced to become a prostitute, which was the only way to pay off the debt of the journey ... She was forced to be out on the streets for 12 hours a day.”