Three people have been arrested in Germany and Slovakia in raids on a criminal syndicate on Monday.
The group are accused of smuggling Vietnamese into European countries for large fees, then forcing them to pay off their debts in massage parlours, brothels and other places, authorities said.
Police in Berlin arrested a Vietnamese woman who lives in the German capital, while a Slovak woman was detained in Bratislava. They are thought to be part of a larger group of suspects, including several Vietnamese, one German and two Slovaks, who brought Vietnamese to Europe on fraudulently obtained visas.
They are accused of demanding payments of between €13,000 and €21,000 for the journeys, which investigators suspect had to be worked off in nail studios, massage parlours and apartments used as brothels.
An investigation involving German police, Slovakian police and Europol found that 250 Vietnamese migrants were brought into the region fraudulently by the group. They got valid Schengen visas on false grounds from various companies connected to the network in Slovakia.
Once the migrants arrived from Vietnam they were taken to Germany by private car. Then deprived of their personal belongings and kept captive until they paid back their debts.
A Vietnamese man is suspected of giving hormone injections to women who worked as prostitutes in Germany to prevent them from becoming pregnant, prosecutors and police said in a statement.
Police found 13 people who were in Germany illegally during Monday's raids in several parts of Germany.