The Hague (AFP) - Interpol and its European counterpart Tuesday warned of growing concerns that radical Islamists could be using illegal people smuggling networks to sneak into Europe or even fund their attacks and activities.

And in a joint report unveiled Tuesday, the two police agencies said those preying on desperate refugees had raked in up to $6 billion (5.2 billion euros) last year. That had turned the migrant flow into one of the largest money-earners for organised crime on the continent.

"Terrorists may use migrant smugglers' resources to achieve their goals," said the report by the global police body Interpol based in Lyon and Europol in The Hague.

"There is a growing concern that illegal immigration routes and networks may be used by radicalised foreign fighters wishing to return to the EU, or by terrorist organisations as a source of funding for their activities."

Although no regular links between criminal gangs and terror groups have been proven "some incidents have been identified involving terrorists who have made use of migratory flows to enter the EU."

It pointed to the November 13 attacks in Paris, after which it was revealed that two of the attackers had entered Europe through Greece as part of an influx of refugees from Syria.

More than a million people snuck into the European Union in 2015, most of them fleeing conflicts in the Middle East as well as poverty in Africa.

"More than 90 percent of all migrants reaching the European Union use the facilitation services of a migrant smuggling network at some point throughout their journey," said Europol director Rob Wainwright.

"This is a clear indication that organised crime networks are profiting from mass migration."

The report calculated that each migrant had "paid an average of $3,200 to 6,500 (EUR 3,000- 6,000)".

"This would result in an average turnover of $5 to 6 billion in 2015."

Both agencies warned that in the future "an increase in the number of migrants trying to reach the EU is expected."

"In Libya alone, around 800,000 migrants are waiting to travel to the EU," it said.