Washington: Terrorists, criminals and foreign fighters are part of the daily refugee flow into Europe, the top NATO commander in Europe told legislators, "masking the movement" of these dangerous elements and heightening the potential for an attack.

In testimony on Tuesday before the Senate armed services committee, US Air Force General Philip Breedlove said IS is "spreading like a cancer" within this mix, "taking advantage of paths of least resistance, threatening European nations" and the United States.

General Breedlove said Russia's actions in Syria have "wildly exacerbated the problem". Despite its public pronouncements, Russia has done little to counter IS but instead has bolstered Syrian President Bashar al-Assad.

Russia and Mr Assad, General Breedlove said, are using migration as a weapon to overwhelm European support structures and break European resolve. He cited the use of barrel bombs, which are unguided weapons and have no military value, against civilians in Syria. The only purpose of these indiscriminate attacks is to terrorise Syrian citizens and "get them on the road" and make them problems for other countries, General Breedlove said.