Terrorists, criminals and returning foreign fighters are part of the refugee flow into Europe.

Terrorists, criminals and returning foreign fighters are part of the refugee flow into Europe, a top NATO commander in Europe said Tuesday.

Speaking before the Senate Armed Services Committee on security challenges in Europe, Air Force Gen. Philip Breedlove outlined challenges ranging from a "resurgent, aggressive" Russia, mass migration of refugees and the threat of the so-called 'Islamic State of Iraq and the Levant' (ISIL) takfiri group.

Breedlove said that the refugee flow is "masking the movement" of destabilizing elements.

“There is a concern that criminals, terrorists, foreign fighters and other extremist organizations will recruit from the primarily Muslim populations arriving in Europe,” he said. “I think that every refugee flow needs to be looked at with an eye towards it could be taken advantage of by those who would seek to do us harm.”

He underscored that the refugee influx in Europe has a “multi-façade” impact on the continent, adding that besides infiltration of terrorists, criminals and returning foreign fighters, the refugee crisis is also bringing about economic challenges and “strains on the social fabric” of the continent.

More than 1 million refugees arrived in Europe last year, fleeing Syria, Iraq and parts of Africa. According to Breedlove.

He suggested the US should not take “any tools off the plate” because of what he see as Moscow threatening American interests and allies.