How is ISIS spreading

NATO's top general said that the current exodus of migrants to Europe is providing cover for terrorists and that the mass migration is allowing ISIS to spread "like a cancer."

Following testimony to the Senate Armed Services Committee, Supreme Allied Commander Air Force Gen. Philip Breed-love told reporters at a Pentagon briefing Tuesday that mass migration spurred by the ongoing conflict in Syria and the threat of ISIS in the Middle East was allowing terrorists seeking to harm the West to have free movement into the continent.

He said the migration "masks the movement of criminals, terrorists and foreign fighters (into Europe)."

"Within this mix, (ISIS) is spreading like a cancer, taking advantage of paths of least resistance and threatening European nations, and our own, with terrorist attacks," he said.

The constant influx of migrants meant Europe was facing "an imminent humanitarian crisis," the United Nations High Commissioner for Refugees, or UNHCR, warned Tuesday.

U.N. warns of humanitarian crisis in Europe 02:49

More than 131,000 migrants had entered Europe in just the first two months of 2016 -- a number that was close to the total for the entire first half of 2015, according to UNHCR figures.

Europe's failure to mount a unified response to the situation meant it now faced a crisis "largely of its own making" as the number of migrants stranded in Greece rapidly increased, the statement said.

A number of European countries, including those along the main Balkan migration route through Europe, recently agreed to tighten border controls to slow arrivals to a trickle.

The move has created a rapidly growing bottleneck of migrants in Greece, a country facing its own severe financial hardships, as the flow of people there from Turkey continues unabated.

Migrants storm Greek-Macedonian border fence 01:53

At Idomeni, a major transit camp on the Greece-Macedonian border, tensions boiled over Monday as migrants were denied permission to cross the border. Macedonian authorities have been letting only a few hundred cross each day, and only Syrians and Iraqis with photo identification.

A group responded to the backlog by ramming through the barbed-wire border fence with a post.

The UNHCR says the number of migrants stuck in Greece had soared to 24,000 by Monday night, with about 8,500 of them stuck at Idomeni.

Elsewhere Tuesday, German Chancellor Angela Merkel said that rather than implementing extra border controls, European countries needed to reinstate the Schengen system of border-free travel within Europe to deal with the crisis.

U.N.: Europe facing 'imminent humanitarian crisis' 01:47

At a news conference with Croatian Prime Minister Tihomir Oreskovic, she stressed that while there was a need to protect the European Union's external frontiers, it was important to reinstate the system of open borders between member states.

"The situation is not yet so that we can be content. Every day we see the pictures from Greece -- we have to get back to the Schengen system," she said, referring to 1985's Schengen Agreement, which guaranteed free movement within Europe. It has been temporarily suspended by some member states and is expected to be amended later this month.

"Greece of course has to protect its borders -- this is not about only protecting the Greek-Macedonian border from the Macedonian side, so that we don't get new routes in the migration flow."

She also urged EU member states to stick to their obligation, made in September, to resettle 160,000 refugees among themselves over two years. So far, only hundreds have been resettled.

Rights groups have cautioned against scapegoating refugees after violence like the deadly coordinated attacks in Paris in November 2015.

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"Significant refugee flows to Europe, spurred largely by the Syrian conflict, coupled with broadening attacks on civilians in the name of the extremist group (ISIS), have led to growing fear-mongering and Islamophobia," Human Rights Watch said in its 2016 World Report.

Breedlove told the Senate Armed Services Committee that alongside the threat posed by extremist organizations in Europe was the potential for unrest from local nationalists opposed to the unprecedented influx of refugees from Syria, Afghanistan, Iraq and unstable parts of Africa.

Fears are they could become increasingly violent, building on the small number of attacks against migrant and refugee populations.

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In his Pentagon appearance, Breedlove also pointed a finger at a "resurgent, aggressive Russia," which "poses a long-term and existential threat to the U.S. and our European allies."

Russia's continued involvement in the Syrian civil war, which Breed-love said had bolstered Syrian President Bashar al-Assad and his allies, has changed the dynamic in the theater and "complicated the problem ... in the air and on the ground."

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The view is compounded by Pentagon reports that Russia is using the nascent, shaky ceasefire in Syria to seize key territory.

Relations between Turkey, the only Muslim-majority member of NATO, and Russia also threatened security, with tensions between the two increasing the "risk of miscalculation or even confrontation."