Defense Secretary Ash Carter is calling Russia a "very, very significant threat," agreeing with an assessment made by top military officials.

A chorus of top military officials have said recently that Russia is the top threat to the U.S.'s national security in comparison to the Islamic State in Iraq and Syria.

At a Pentagon press conference, Carter said he agreed with their assessment that Russia is an existential threat to the U.S. "by virtue simply of the size of the nuclear arsenal that it's had."

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"Now, that's not new," he added. "What's new — and I think also that they were pointing to and where I agree with them — is that for a quarter century or so, since the end of the Cold War, we have not regarded Russia as an antagonist.

"Vladimir Putin's Russia behaves, in many respects, as — in some respects and in very important respects, as an antagonist. That is new. That is something, therefore, that we need to adjust to and counter," Carter continued.

He also laid out the Pentagon's strategy in countering Russia — an approach he called "strong and balanced."

"The strong part means we are adjusting our capabilities qualitative and in terms of their deployments, to take account of this behavior of Russia," he said.

"We are also working with NATO in new ways, a new playbook, so to speak, for NATO ... more oriented towards deterrence on its eastern border and with hardening countries [on] the borders of Russia, NATO members and non-NATO members, to the kind of hybrid warfare influence or little green man kind of influence that we see associated with Russia in Ukraine," he said.

The "little green man" refers to Russian forces in green military uniforms acting in Ukraine.

However, Carter said the U.S. would continue to work with Russia on areas where both countries' interests align.

"The balanced part is we continue to work with Russia, because you can't paint all their behavior with one brush. There are places where they are working with us: in counterterrorism in many important respects, in some respects, with respect to North Korea, in some respects with respect to Iran and elsewhere," he said.

"So where Russia sees its interests as aligned with ours, we can work with them and will continue to do that," Carter added.

Russian troops last year invaded Ukraine and annexed the Crimean peninsula, prompting the U.S. and Western allies to impose several rounds of sanctions on Moscow.

Despite a ceasefire negotiated late last year, Ukrainian forces are locked in heavy battle with separatists in the eastern part of the country, who U.S. and Western officials say are armed and trained by Russian forces.

"We'll continue to hold open the door so that if either under Vladimir Putin or some successor of his in the future, there's a leadership that wants to take Russia in the direction that, I believe, is best for Russia," Carter said.

He said a new direction would be "not one of confrontation with the rest of the world and self-isolation, which is the path they're on now, but better economic and political integration with the rest of the world."