Donald Trump says the United States has fallen behind and must return to the “top of the pack” in nuclear weapons capability. The president on Thursday called a recent report of a missile deployment and arms control violation by Russia a “big deal” that he would raise with Russian President Vladimir Putin if the two meet.

Moscow has a decidedly different view. From the Kremlin’s perspective, Washington has unfairly held back Russian nuclear might through arms control treaties while developing its own new weapons, wrecking the nuclear balance and fear of “mutually assured destruction” that staved off Armageddon during the height of the Cold War.

Trump’s latest attempt at nuclear tough talk comes one month after he suggested cancelling U.S. sanctions on Russia in exchange for a nuclear arms reduction deal. Putin’s spokesman and members of parliament quickly rejected that idea, saying Moscow would not sacrifice nuclear security for sanctions relief. But Trump clearly struck a sore spot in long-simmering U.S.-Russian relations that precede the sanctions, imposed in 2014 when Russia illegally annexed Crimea: strategic parity.

“It’s not the missile that is scary but rather the [possibility] that a key agreement is being violated.”

Russia has been pushing back against what it perceives as an imbalance in recent years, and not just through angry rhetoric. U.S. administration officials told the New York Times last week that Russia has secretly deployed a new cruise missile that violates the 1987 Intermediate-Range Nuclear Forces Treaty, a landmark deal to limit nuclear weapons. Russian officials have denied even the existence of such a missile, while also claiming that the INF treaty is unfair and that the United States is actually the one violating it.

The report about the new missile has reignited long-standing arguments over nuclear weapons development. After agreeing to make huge cuts in the New Strategic Arms Reduction Treaty in 2011, Washington and Moscow have increasingly reverted to antagonism in this arena. Trump has called New START a “bad deal,” and Putin last year suspended a different U.S.-Russian agreement to dispose of weapons-grade plutonium. Both presidents have called for an expansion of their nuclear forces.

Despite Trump’s warm words for Putin, growing complaints from both parties about their nuclear capabilities could put the brakes on any rapprochement, or even spark a renewed arms race.

“It’s not the missile that is scary but rather the [possibility] that a key agreement is being violated,” Dmitry Stefanovich, a nuclear arms control expert at the Russian International Affairs Council, told VICE News. “Russia and the United States need to preserve this treaty, because if it’s broken up, the United States will have to put serious offensive systems in Europe, leaving the Kremlin open to attack. [Russia] would have to create a threat to the capitals of U.S. allies in Europe.”

Reports of the deployed cruise missile have emerged as Russia makes an aggressive push to expand and modernize its military, including its strategic nuclear forces, which Putin said late last year need to be strengthened. This week, defense minister Sergei Shoigu said the military had gotten 41 intercontinental ballistic missiles in 2016, among other upgrades, and that three regiments would receive new ICBMs this year. (Following accusations of Russians hacking the U.S. election last year, he also acknowledged for the first time a unit of information-warfare troops that he said were spreading “intelligent, effective propaganda,” but he didn’t go into greater detail.)

The cruise missile in question is the SSC-8 (9M729 is believed to be the Russian name for it), one of the intermediate-range land-based missiles banned by the treaty. Although the United States has been flagging the missile’s development as a violation since 2013, the Russian design bureau that builds guidance for cruise missiles made a statement in 2015 that the 9M729 had completed state trials, according to the respected blog Arms Control Wonk.

Now U.S. administration officials have said the weapon came online late last year. Russia has two battalions of 9M729 launchers with about two dozen nuclear-tipped missiles each, one at a testing ground and the other at an operational base. These launchers closely resemble those of Russia’s Iskander, a short-range nuclear-tipped missile allowed under the INF treaty, making it hard to verify violations.

The Kremlin unsurprisingly rejected news of the missile’s deployment. Vladimir Putin’s spokesman told state news agencies on February 15th that “no one has officially accused Russia of violating the INF treaty,” claiming that Moscow “remains committed to its international obligations.” A foreign ministry spokesman called the accusations “absolutely baseless,” and it put a copy of the article — covered with a red “fake” stamp — on a site it launched this week to counter what it claims is false information in established Western media outlets.

Igor Korotchenko, director of the Center for World Arms Trade Analysis and a prominent military affairs pundit, told Argumenty i Fakty newspaper there was no “evidence of the deployment or even existence of this land-based cruise missile.” And retired Lt. Gen. Yevgeny Buzhinsky, who participated in the INF special verification commission in earlier years, told VICE News that the United States had failed to provide “hard evidence” like photographs or flight-test data to prove the missile’s existence.

“They can write anything just to accuse Russia of something and attack Trump and his circle,” he said of the report about the missile’s deployment. “It’s just not serious.”

Nikolai Sokov, a senior fellow at the James Martin Center for Nonproliferation Studies, offered a another possibility. He said the new missile could be an “R&D program that kept going without attracting attention of the political leadership.”

“There are many examples (I witnessed a couple myself) when the defense industry went on with a program coming close to violation (if not violating) agreements just because they do not read treaties,” Sokov said.

The New York Times report quoted only anonymous sources, but publicly available evidence of the missile’s existence is mounting. Since the report, U.S. defense officials have said Russia is in violation of the INF treaty and that the new missile has “moved around” in recent months. The state department said Russia has broken the treaty ban on cruise missiles with a range of 500 to 5,500 kilometers.

Buzhinsky argued that the United States had itself breached the INF treaty by activating a missile defense shield last year in Romania, touching on a regular Russian refrain. Despite U.S. assertions that the missile shield is directed against the growing threat from rogue states like Iran and won’t protect against Russian missiles, Moscow has long contended its true target is Russia. This fear stems from the fact that the Aegis Ashore anti-ballistic missile system deployed in Romania is similar to a system on U.S. warships that can fire Tomahawk cruise missiles. Although there are no Tomahawk missiles in Romania, Buzhinsky said the launchers there could be repurposed to fire them, moving U.S. nukes much closer to Russia than the Kremlin would like to tolerate.

Last year, Putin called the missile shield a “threat” and said Russia was “obliged to take action in response to guarantee our security.” He later said the United States sought to “neutralize the strategic nuclear potential” of Russia. (After a decade of talks, the United States has also broken ground on a second missile shield site in Poland due to come online in 2018.)

A number of other U.S. military developments have, in Moscow’s opinion, also disrupted the nuclear balance: Korotchenko claimed modern U.S. armed drones are in violation of the INF treaty, and Russian arms control experts have also accused the United States of keeping banned intermediate-range missiles for testing purposes.

Moscow’s many complaints stem from a deeper concern with the treaty’s feasibility, since Russia has a host of countries near its borders with nascent or well-developed intermediate-range missile arsenals, including China, India Iran, North Korea and Pakistan. The basic idea is that the treaty allows the United States to push its advantage in sea-based cruise missiles and missile defense while Russia is hindered from developing its most effective weapons.

“The Americans don’t need [intermediate-range missiles] to protect their territory, but Russia borders countries that have these missiles, and yet we don’t have them,” Buzhinsky said.

According to Stefanovich, although Moscow has never officially confirmed the 9M729, it has the know-how to develop it. He said the Kremlin could deploy the missile to try to force Washington to come back to the negotiating table and address its concerns with the INF treaty.

“Such logic seems to fit the pattern of Russian policy in … defense and security domains: raise concerns, double the stakes, get down to negotiations,” he said.

But while negotiations have yet to materialize, the signs of a nascent arms race have. The United States, Russia, China, and India are all reportedly developing hypersonic weapons that can travel at or above Mach 5. These weapons would be able to penetrate missile defenses and greatly reduce the time a country has to decide whether to launch a retaliatory strike, which would destabilize the nuclear balance. The United States, Russia, and China have also been opening a new front with space-based weapons. Russia in particular has reportedly tested missiles meant to destroy satellites and may even have kamikaze satellites for the same task.

Nikolai Sokov said Russia’s launch of new Kalibr cruise missiles at targets in Syria last year shows the United States may be losing its monopoly on high-precision long-range conventional weapons.

“This is a reason to start negotiations on these weapons, the same as on missile defense, where Russians are quickly closing the gap, too,” he said. “Otherwise we face an unrestricted arms race in two very dangerous categories.”

Putin declared last year that new Russian weapons would be able to pierce missile defense, perhaps referring to some of these programs. A state television broadcast last year even suggested that Russia was developing an underwater nuclear-armed drone.

Trump, for his part, has reportedly welcomed the challenge. “Let it be an arms race,” he told “Morning Joe” co-host Mika Brzezinski in December. “We will outmatch them at every pass and outlast them all.”

But Stefanovich cautioned that while the current arms competition is worrying, it is nowhere near Cold War levels.

“There is threat, and it’s increasing, but it hasn’t surpassed the critical level,” he said. “It’s not in the interests of Russia, since the United States has significantly bigger potential.”