The top NATO commander in Europe says terrorists, criminals and foreign fighters are part of the daily refugee flow into Europe.

In congressional testimony Tuesday, U.S. Air Force Gen. Philip Breedlove says the refugee flow is "masking the movement" of these dangerous elements.

He says the Islamic State extremist group is "spreading like a cancer" within this mix. He says that the Islamic State group is taking advantage of paths of least resistance, threatening European nations and the United States with terrorist attacks.

Breedlove tells the Senate Armed Services Committee that the refugee flow has to be monitored carefully.

The general says Russia's actions in Syria have "wildly exacerbated the problem." Breedlove says Russia and Syrian President Bashar Assad are using migration as a weapon to break European resolve.