The Hague: European police have arrested six suspected members of a Finnish-led gang who trafficked women for sex, seizing almost three million euros in assets in four countries, officials said Wednesday.

The operation codenamed "Webmaster" also shut down 16 websites advertising sexual services and led to the arrest of the alleged ringleader based in Marbella in Spain, the EU's police agency Europol said.

Two other suspects were also arrested in Spain and others in Malta, Romania and Finland.

"The suspected leader of the criminal group has carried out criminal activities in at least 15 countries," said the Hague-based Europol, which coordinated the operation together with EU legal agency Eurojust.

"He used intermediaries to channel criminal proceeds to international multi-currency bank accounts" and also used small amounts of crypto currencies, it said.

The women were mainly Nigerian and worked as prostitutes in Finland and Sweden, Eurojust said in a separate statement.

Police have now frozen the suspects' bank accounts in 12 different countries including in Hong Kong, as well as seizing luxury cars, jewellery and about 30,000 euros in cash, Europol said.

Spanish authorities opened their investigation three years ago into the group, which is said to have made some 40 million euros in criminal profits since 2010.