WASHINGTON – The Trump administration will halt U.S. compliance with a nuclear arms control treaty with Russia, Secretary of State Mike Pompeo said Friday, citing Moscow's "brazen" violations of a pact that has been a cornerstone of nonproliferation since the Cold War.

"For almost six years, the United States has gone to tremendous lengths to preserve this agreement," Pompeo said. He said the Kremlin's repeated denials that it has developed a covert missile system in violation of the treaty has left America and its European allies at immense risk.

"Russia has jeopardized the United States' security interests, and we can no longer be restricted by the treaty while Russia shamelessly violates it," Pompeo said.

The U.S. suspended compliance with the Intermediate-Range Nuclear Forces Treaty, known as INF, on Feb. 2 and began a six-month formal withdrawal process, Pompeo said. That gives Moscow time to reverse course – even as the Trump administration begins to look at developing and deploying new intermediate-range missiles.

"We will move forward with developing our own military response options and will work with NATO and our other allies and partners to deny Russia any military advantage from its unlawful conduct," President Donald Trump said in a statement on Friday.

Supporters of the move said it was long overdue after years of trying to cajole Russia back into compliance proved fruitless.

“The Russian government has had endless opportunities to change their bad behavior and ... has proven its disinterest in doing so," said Sen. James Risch, R-Idaho, chairman of the Senate Foreign Relations Committee. "The time has come to set the treaty aside and develop alternative avenues toward the security the treaty once provided.”

The administration's decision will free the United States to confront other emerging security threats, principally China's buildup of intermediate-range missiles.

But critics said abandoning the 32-year-old treaty could spark a new arms race, undermine American credibility and put Europe at risk of Russian aggression.

“Russia’s brazen noncompliance with this treaty is deeply concerning, but discarding a key pillar of our nonproliferation security framework creates unacceptable risks," said House Speaker Nancy Pelosi, D-Calif. "The administration should exhaust every diplomatic effort and work closely with NATO allies over the next six months to avoid thrusting the United States into a dangerous arms competition."

Though experts do not dispute Russia's violations, proponents of arms control argue the U.S. decision will allow Russia to be more overt and aggressive.

"This really changes the dynamic where Russia could deploy systems are that are much more offensive and that upsets the security balance in Europe, because it can hold at risk a lot of the countries who are NATO allies and partners," said Rachel Ellehuus, deputy director and senior fellow with the Europe Program at the Center for Strategic and International Studies, a Washington think tank.

Russia will now be able "to turn the narrative on the United States and NATO and essentially say 'We need to defend ourselves, and the way we defend ourselves is by deploying these systems in our neighborhood,'" Ellehuus said.

Kremlin spokesman Dmitry Peskov told reporters Friday that Russia greeted the decision with "much regret" and blamed the U.S. for the INF's collapse. He said Washington has been “unwilling to hold any substantial talks” to save it.

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The INF Treaty, signed in 1987 by President Ronald Reagan and Soviet President Mikhail Gorbachev, bars the U.S. and Russia from deploying ground-launched ballistic and cruise missiles with ranges of 310 to 3,400 miles. The agreement led to the elimination of nearly 3,000 missiles with nuclear and conventional warheads and contributed to the end of the Cold War.

But the U.S. has long accused Russia of violating the pact – stretching back to the Obama administration. European leaders also have accused Russia of cheating on the agreement, even as Kremlin officials insisted Russia was in compliance.

In a statement Friday, NATO strongly backed the Trump administration's move and said Russia was to blame for the U.S. decision.

"The United States is taking this action in response to the significant risks to Euro-Atlantic security posed by Russia’s covert testing, production, and fielding of 9M729 ground-launched cruise missile systems," NATO said in its statement. "Allies fully support this action."

It's not clear yet how European allies will respond. It's possible that NATO would allow the U.S. to deploy ground-launched missiles in Europe, but that step probably would provoke Russia even more and would risk a public backlash across the continent.

"NATO continues to closely review the security implications of Russian intermediate-range missiles and will continue to take steps necessary to ensure the credibility and effectiveness of the Alliance’s overall deterrence and defence posture," NATO said Friday. "We will continue to consult each other regularly with a view to ensuring our collective security."

Ellehuus said the Europeans could move in one of two directions. "They could argue 'Gosh, we need a stronger nuclear deterrent,' and then we’re in a new arms race," she said. "Or they could say: 'You know what? The U.S. doesn’t have our back anyway; we want all U.S. nuclear weapons out of Europe.'"

A senior administration official, who was not authorized to speak on the record, said the Pentagon has started on research and development of intermediate range missiles. But the U.S. is not close to making any decisions about whether and how such weapons would be deployed, the official said, adding that the administration is focused on conventional, not nuclear, missiles.

“We are only looking at conventional options at this time,” the official said. “Nothing the United States is currently looking at is nuclear in character.”

This official also emphasized that any decisions about deployments would be made in close consultation with NATO allies. He said Friday’s decision was not a signal that the U.S. would pivot toward an arms race with China, although he noted that China has more than 1,000 intermediate-range missiles.

And in discussing the INF withdrawal decision last year, Trump National Security Adviser John Bolton noted that China was not bound by the treaty and argued that Beijing represents a growing threat to the U.S.

"Exactly one country was constrained by the INF Treaty: the United States,” Bolton said while in Moscow in October. In the meantime, Bolton said, China, North Korea and Iran “are free to do whatever they want” and have made “very substantial strides” in developing intermediate-range missiles.

Some experts say the administration's withdrawal from the pact also jeopardizes a second nuclear arms control agreement with Russia known as New START that expires in 2021. The Russians have asked the U.S. to begin negotiations on an extension of that agreement, but so far the Trump administration has not agreed to do that.

Asked if the Trump administration was committed to extending New START, Pompeo did not answer directly. He said Trump was committed to reducing the proliferation of nuclear weapons but said, "It does no good to sign an agreement if a party is not going to comply with it."

Experts say Russia is in compliance with New START, and the administration has not accused Russia of cheating on the pact.

Steven Pifer, a fellow at Stanford University's Center for International Security and Cooperation, said the Trump administration's refusal to begin those complicated negotiations is worrisome and a sign it may want to see that treaty expire.

"There’s a very early and an unfortunate possibility that in 2021, for the first time in five decades, you have no negotiated limits on U.S. or Russian nuclear forces," Pifer said.

He said that would severely undermine global non-proliferation. The U.S. would no longer have credibility, he said, to press North Korea and other rogue states to give up their nuclear arsenals if America has abandoned its own arms-control agreements.

Indeed, Iran's Foreign Minister Mohammad Javad Zarif used Friday's news to jab the U.S. as an unreliable negotiator, a theme he has hit on since Trump's decision to pull out of the Iran nuclear accord, known as the JCPOA.

"Yet another withdrawal from an accord by the Trump administration; this time the #INFTreaty," Zarif tweeted. "It's not just the #JCPOA or Iran: Seems this clique is allergic to anything w/ US signature on it. Message: Any deal with US govt is not worth the ink; even treaties ratified…"

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Contributing: David Jackson and Kim Hjelmgaard