The FIFA World Cup is known for being a month-long soccer fest with fans pouring into the host country from all over the world. But Nigerian sex traffickers could take advantage of Russia’s relaxed visa rules during the tournament, advocates say.

In fact, the ruse has already been trialled: Alternativa, a Russian anti-slavery group, claims that up to 30 women were trafficked into Russia during last summer’s Confederation Cup—a smaller tournament among continental champions played a year before the World Cup in the host nation. Alternativa says it expects the country “to face the same problem” during the World Cup, Reuters reports.

Russia implemented its visa-free regime for the Confederations Cup and will also have it in place during the World Cup which lasts from June 14 till July 15. Foreigners will be able to enter Russia during that period while holding only a FIFA-backed fan pass and valid match tickets. Alternativa’s claims have been corroborated by Nigeria’s national anti-trafficking agency which says it is aware of plans by traffickers.

Nigeria’s long-running sex trafficking problems have become even more rampant in recent years as traffickers have typically lured women on dangerous trips across the Mediterranean to Italy often with the promise of a better life of legitimate work and education. The empty dreams are hardly ever fulfilled though: four of five Nigerian women that arrive in Italy are forced into prostitution, the International Organization for Migration says.