The European Union's top prosecutor said she has been told that smugglers' boats bringing migrants across the Mediterranean to Europe are also carrying terrorist fighters from Islamic State.

Michele Coninsx, head of the EU's judicial co-operation agency Eurojust, told reporters she received the information as part of Eurojust's efforts to help EU nations jointly respond to illegal immigration, terrorism and cybercrime.

Ms Coninsx said Eurojust's co-ordination efforts are ongoing and she could not divulge what EU nations have told the agency.

She said it is not yet clear what problem the reported infiltration of Islamic militants may pose for European law enforcement.

But she said groups such as Islamic State are also using proceeds from people-trafficking to fund terrorism.

Eurojust, she said, is one of several EU agencies mobilised to shut down traffickers' operations.

"We're going after the criminals. We're going after the money," said Ms Coninsx, a career prosecutor from Belgium who is also Eurojust's chief official for terrorism investigations.

"It is an alarming situation because we see obviously that these smugglings are meant to sometimes finance terrorism, that these smugglings are used sometimes to have and ensure exfiltrations and infiltrations of members of Islamic State."

Paying large sums of money for a shot at reaching Europe, tens of thousands of people have left Libya in unseaworthy and overcrowded boats or dinghies over the past two years. An unknown number have drowned. This year, about 70,000 migrants have been rescued, many by an EU-led naval operation.

Ms Coninsx said an Italian official told Eurojust three weeks ago the flow of migrants has risen fivefold.

Often the smugglers mingle with the migrants in the hope they will not be caught - although dozens have been arrested in Sicily and other parts of southern Italy.