Pope Francis has warned against the risk that militants could slip into Europe under cover of a huge wave of refugees fleeing war in Syria but also said the migrant crisis could help reawaken the continent's conscience.

In an interview published on Monday with the Portuguese Catholic broadcaster Radio Renascenca, the pope referred to the risk that Islamic State, which has killed Christians and other minorities in the Middle East, could launch attacks in Europe.

"It's true, I also want to recognise that, nowadays, territorial security conditions are not the same as they were in other periods [of mass migration]," he said in the interview.

"The truth is that just 400 kilometres (250 miles) from Sicily there is an incredibly cruel terrorist group. So there is a danger of infiltration, this is true."

Security specialists have said the risk that militants could be smuggled into Europe in this way is small.

Asked if Rome could be the target of an attack, the pope said: "Yes, nobody said Rome would be immune to this threat. But you can take precautions."

Islamic State militants have made threats against Catholic targets in Rome that have been widely reported and security has been stepped up in the Vatican City and other religious sites in Italy that draw many pilgrims and tourists.

On Sept 6, the pope appealed to every Catholic parish, religious community and sanctuary in Europe to take in a family of refugees, saying he would set the example by hosting two families in parishes inside the Vatican..

"These poor people are fleeing war, hunger, but that is the tip of the iceberg. Because underneath that is the cause; and the cause is a bad and unjust socioeconomic system," he said in the interview.

Asked if the refugee crisis could be a positive occasion for Europe and a re-awakening of the continent's conscience, the pope said "it could be".