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

In testimony Tuesday before the Senate Armed Services Committee, U.S. Air Force Gen. Philip Breedlove said ISIS is "spreading like a cancer" within this mix, "taking advantage of paths of least resistance, threatening European nations" and the United States.

Breedlove later told reporters that he has asked for more U.S. forces to be permanently based in Europe, which is facing threats from a more aggressive Russia as well as ISIS.

He provided no details in his first public comments on the request, but a senior U.S. official said Breedlove has asked for more Army troops that could be ready to respond quickly to any crisis or military need. The official, who was not authorized to discuss the matter publicly and requested anonymity, would not say how many troops Breedlove had requested.

The Pentagon has steadily decreased the number of U.S. forces in Europe over the last several years, choosing to shift focus to the Asia Pacific region while fighting continues in the Middle East. But the growing threats from Russia and ISIS have raised questions about those decisions.

In response to a question about whether ISIS will continue to infiltrate refugee flows, Breedlove said, "I think that they are doing that today."

Those fears fueled legislation that would crack down on Syrian and Iraqi refugees seeking to enter the United States. A House-passed bill would require new FBI background checks and individual sign-offs from three high-ranking federal officials before any refugee from Syria or Iraq could enter the country.

The legislation cleared the House in November in the aftermath of the Paris attacks. It received 289 votes, a veto-proof margin that included 47 Democrats — despite President Barack Obama's opposition. But in January, Senate Democrats blocked the legislation, saying it reflected Republican presidential candidate Donald Trump's call to bar Muslims from coming to the United States.

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

Russia and Assad, Breedlove said, are using mass 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 terrorize Syrian citizens and "get them on the road" and make them problems for other countries, Breedlove said.

In the wake of Syria's civil war and the rise of ISIS, the people fleeing Syria and Iraq were what Breedlove described as "legitimate" refugees. They were escaping civil war, terrorism and unresponsive governments. But that's changed in recent months, he said.

"This criminality, the terrorists, and the returning foreign fighters are clearly a daily part of the refugee flow in Europe," Breedlove said. He added that reports suggest that about 6,000 foreign fighters have left to join the fight in Syria and Iraq, and that about 1,500 of them have returned to Europe.

The International Organization for Migration said Tuesday that almost 130,000 refugee — more than 2,000 each day — have reached Europe by sea since the beginning of this year and 418 have died trying, most drowning in the cold, rough waters. The number of arrivals in the first two months of 2016 is more than 11 times as many as the same period last year.

The organization estimated that last year's total of more than 1 million seaborne refugee arrivals likely will be surpassed in 2016, possibly before the end of the summer.

Breedlove said forcing the flow of refugees into Europe fits into Russia's goal of using nonmilitary means to create divisions in the NATO alliance and the European Union.

"What we have seen is that this is putting great pressure on the nations of Europe," he said.

In addition to the dangers posed by terrorists in the refugee mix, Breedlove warned that opposition from local nationalists in Europe to the large influx of refugees could lead to violence.