At least one suspect in a Europe-wide pimping and sex trafficking ring has been arrested in Malta, the EU's police agency said on Wednesday.

Officials said European police operating in four different countries arrested six suspected members of a Finnish-led gang and seized almost €3 million in assets.

The ring’s alleged ringleader was arrested in Marbella in Spain, with two more suspects caught in Spain and others in Malta, Romania and Finland.

The operation codenamed "Webmaster" also shut down 16 websites advertising sexual services, Europol said. 

Two houses in Malta were raided and searched as part of the operation. It is understood the arrests happened in March, but authorities were only able to release the information on Wednesday.

"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 in cash, Europol said.

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