With the proliferation of fake Syrian and Iraqi passports produced by counterfeiters from the Islamic State (ISIS), it is "virtually impossible'' now for security officials to detect potential terrorists from among the refugees, according to a senior British intelligence official.

"Islamic State is skilfully exploiting the migrant crisis to smuggle terror cells from Syria into major European countries such as the U.K.," the official said.

"Jihadists travel to Raqqa to meet up with ISIS commanders, where they receive training and new passports. They then make their way back to Europe posing as migrants with new identities, making it virtually impossible for security officials to detect potential terrorists among those fleeing persecution."

He said the terrorists disguised as refugees enter the continent often via Turkey and Greece, the same route taken by at least two of the Paris attackers, including one whose fake Syrian passport was left at the scene of the Stade de France suicide bombings.

All members of the terror cell who murdered 130 people are believed to have trained with ISIS at its stronghold in Raqqa before making their way into Europe, he said.

The extremist group has announced its intention to exploit the refugee crisis early last year, when one of the group's affiliates wrote an essay claiming that migrant boats could be used to cause "pandemonium" in Europe. The main route being used by smugglers then was between Italy and Libya.

French Prime Minister Manuel Valls said ISIS has been "taking advantage" of the huge influx of asylum seekers as he called for the strengthening of Europe's internal borders.

Authorities have admitted that verifying identities has been difficult with the volume of arrivals and that compulsory registrations and fingerprinting slipped during the summer peak.

"On the issue of fake documents, many are sold in Middle East markets and we know how difficult it is to identify them with good machines, in calm conditions, not when you have 4,000 arriving in a day," the Telegraph quoted citizen protection minister Nikos Toskas as saying.

However, a report released last week by Europol said there was no concrete evidence of terrorists "systematically using the flow of refugees to enter Europe unnoticed."

Apart from the ISIS training camps in Syria, it said there are several smaller ISIS camps in the EU and Balkan countries where recruiters use sport and survival training to test aspiring members.

"Without reliable intelligence on the intentions, activities and contacts and travels of known terrorists it is nearly impossible to exactly predict when and where the next terrorist attack will take place, and what form it will take," Europol said.

The U.N.'s former High Commissioner for Refugees, António Guterres has, however, cautioned that spreading fear and distrust of refugees played into ISIS propaganda.

He added that the fake Syrian passport in the name of Ahmad Al-Mohammad was "left to be seen" as part of an ISIS strategy to "put refugees in the spotlight."

"It is not the refugee outflows that cause terrorism, it is terrorism, tyranny and war that create refugees," Guterres added.