Although he earlier encouraged Catholic churches in Europe to take in refugees from the strife-torn parts of the Middle East, Pope Francis has warned European countries that the present migrant crisis may be used by terrorist groups like the Islamic State of Iraq and Syria (ISIS) to sneak into the continent.

In an interview published on Monday with the Portuguese Catholic broadcaster Radio Renascenca, the leader of the Roman Catholic Church said that while European countries should be compassionate with the refugees, they should also carefully guard their borders against the ISIS, which has been killing and persecuting Christians.

"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)," the Pope said in his interview.

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

Pope Francis also acknowledged that even Italy—where the Vatican City, which he leads, is located—is vulnerable to possible attacks by terrorists who may use the migrant crisis as an opportunity to illegally enter Europe.

"Yes, nobody said Rome would be immune to this threat. But you can take precautions," the Pope said.

Similarly, Republican presidential aspirant and former Arkansas governor Mike Huckabee has also warned the US government on accepting more refugees, saying some of them may have "nefarious purposes."

"This idea that we have an obligation to receive vast numbers of people ... we have a real obligation to make sure that we protect the sovereignty of the United States," Huckabee said.

"That's not a lack of Christian charity. It's the essence of charity, to provide for needs, but not to put your own children at risk, if what you're importing could be people who have a nefarious purpose for wanting to be here," he added.