(VOVworld) - The EU border agency Frontex says the Islamic State may have stolen "tens of thousands" of blank passports that it could use to smuggle its fighters into Europe as refugees.

Germany is Europe's top destination for refugees, with 964,574 arriving in 2015 (Photo: Christof Stache/ AFP)

IS could have acquired the stolen travel documents in areas of Syria, Iraq and Libya it now controls. The passports could be handed out to would-be attackers to enter the European Union as asylum seekers.

IS has already launched a money-making operation with the fake documents, selling them on the black market where they fetch up to 1,500 euros each.

European authorities have repeatedly warned of the potential threat posed by refugees travelling with counterfeit documents.