Russia is nearing deployment of a new missile capable of targeting all of Europe with nuclear or conventional warheads, according to defense officials.

Disclosure of the new SSN-30A missile threat comes as Defense Secretary Ash Carter on Thursday warned that Moscow under Vladmir Putin is reemerging as an existential security threat.

Russia "is a very, very significant threat," Carter told reporters at the Pentagon. "And I think a point that they've made, but I would certainly make: Russia poses an existential threat to the United States by virtue simply of the size of the nuclear arsenal that it has."

Regarding the SSN-30A, designated as the "Kalibr" missile, Pentagon officials said the new naval weapon can be equipped with both nuclear and conventional warheads and can reach most of Europe when fired from ships in the Black Sea.

The longer-range version of the Kalibr can reach between 620 and 923 miles. A shorter range version can hit targets at distances of up to 180 miles.

A cruise missile variant also is being developed that officials said appears to violate the 1987 Intermediate-Range Nuclear Forces (INF) Treaty.

A test of the Kalibr was conducted earlier this month, officials said.

The new supersonic missile is capable of being used to strike targets both at sea and on land.

"This system is about ready to be deployed," said one official who voiced concerns for U.S. interests and those of allies in Europe. "It allows the Russians to cover most of Europe from the Black Sea on naval vessels.

"They can hold all of Europe at risk," the official said. "This is like putting SS-20s in Europe again."

SS-20 intermediate-range nuclear missiles were withdrawn under the INF treaty in the 1980s.

Additionally, naval vessels equipped with SSN-30s also could be deployed from the Black Sea to Kaliningrad, the Russian enclave on the Baltic Sea bordering Poland and Lithuania.

Russian officials have threatened to deploy short-range Iskander missiles in Kaliningrad, in response to U.S. missile defense deployments in Europe.

Mark Schneider, a former Pentagon nuclear strategy expert, said the Kalibr is a capable, supersonic, very accurate nuclear and conventional missile.

The missile is expected to see "a very widespread deployment" on both submarines and surface ships, including the new type 885 Yasen class submarine, older submarines and cruisers, and newer models of destroyers.

"Thanks to 25 years of bad decisions on nuclear deterrence, we have no comparable capability," Schneider said.

"The Obama administration in the 2010 Nuclear Posture Review Report announced the elimination of the nuclear-armed version of the Tomahawk submarine launched cruise missile," he added. "If Putin attacks one of the vulnerable NATO states, the Kalibr will likely be one of the main threat weapons used to try to deter a NATO counter attack. Granting Putin a monopoly on almost all types of tactical nuclear weapons is plain stupid."

Carter said Russia under President Vladimir Putin has become a new "antagonist," following the military takeover of Ukraine’s Crimea and continuing covert arming of pro-Russian rebels in eastern Ukraine.

"Vladimir Putin's Russia behaves, 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," the defense secretary said.

The Pentagon has adopted a policy called "strong and balanced," Carter said.

"The strong part means we are adjusting our capabilities qualitatively and in terms of their deployments, to take account of this behavior of Russia," he said, adding that the Pentagon has begun working with NATO states in new ways, such as deterring Russia and hardening borders near Russia.

Efforts also are being taken to counter Moscow’s use of "hybrid warfare," the use of both military forces and information warfare.

The harder line approach is being balanced by continued cooperation with Moscow on counterterrorism, North Korea, and in some aspects Iran.

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

He added that the United States "would hold open the door" to future cooperation with Russia, should its approach change.

In his remarks, Carter bolstered earlier expressions of concern from Marine Corps Commandant Gen. Joseph Dunford, the next chairman of the Joint Chiefs of Staff, who said during a Senate hearing last month that Russia "presents the greatest threat to our national security."

NATO commander Gen. Philip Breedlove told PBS July 29 that Russia poses a greater danger than the Islamic State terrorist group.

Breedlove said the United States sought to make Russia a partner for the past 20 years. Instead, Moscow has "used force to change internationally recognized boundaries" by occupying Crimea and moving military forces in eastern Ukraine.

"And this is a nation that possesses a pretty vast nuclear inventory, and talks about the use of that inventory very openly in the past," Breedlove said.

"And so what I think you see being reflected is that we see a revanchist Russia that has taken a new path towards what the security arrangements in Europe are like and how they are employed. And they talk about using, as a matter of course, nuclear weapons. For that reason, these senior leaders, I believe, see that as a major threat."

Asked if U.S. support for NATO allies near Russia could trigger a war, Breedlove said: "Well, the best way not to have a war is to be prepared for war. So, we’re in there now, training their soldiers."