ISIS is smuggling extremists into Europe using boats carrying migrants across the Mediterranean from North Africa, an adviser to the Libyan government has said in an interview.

Abdul Basit Haroun told the BBC that militants with the group Islamic State in Iraq and Syria are hiding among refugees who, for a number of years, have been boarding boats in Libya destined for the Italian coast.

Haroun said he gathered information from conversations he had with the smugglers themselves. He said ISIS has used the boats believing "European police don't know who is from [Islamic State] and who is a normal refugee or not."

These militants often sat separately from the other migrants, he said.

If the claims are true, they represent a significant challenge for law enforcement agencies.

Officials from the agency Frontex, which manages EU borders, have previously warned that ISIS could be infiltrating the boats. However, they add there is no hard evidence supporting the claims.

Western Libya is the main launch pad for smugglers sending boatloads of migrants from Africa and the Middle East toward Italian shores.

That region in Libya is controlled by a rival government set up after an armed group seized Tripoli in August.

There has been a sharp increase in the number of migrants since the 2011 uprising in Libya that overthrew dictator Moammar Ghadafi brought greater instability to the country.

The United Nations estimates some 60,000 migrants have made the journey by boat to Italy so far this year, with about 1,800 losing their lives at sea.

With files from Reuters