The United States and Russia may be on the verge of a new arms race in Europe, decades after the missile crisis that shook the Old Continent in the waning years of the Soviet Union.

President Donald Trump has repeatedly said he would like to build warmer ties with Moscow, but in Europe tensions have been high since Russia annexed Crimea and began to foment separatism in Ukraine.

NATO has deployed reinforcements to the Baltic states and Poland on its eastern flank, and the United States and European Union have imposed sanctions on Russians implicated in the unrest.

Now, according to the New York Times, Moscow has secretly deployed an operational ground-launched cruise missile unit of a type that contravenes a 1987 US-Russia arms control treaty.

US officials have not confirmed the anonymously-sourced report, but the State Department expressed concern that Russia was in any case already in breach of the Intermediate-Range Nuclear Forces Treaty.

If confirmed, the move would be a flagrant violation of the treaty negotiated by president Ronald Reagan and Soviet leader Mikhail Gorbachev that banned intermediate-range missiles from US and Russian arsenals, and led to the destruction of 2,700 missiles.

US Secretary of Defense James Mattis, speaking in Brussels on Wednesday, said the NATO transatlantic military alliance would "defend ourselves if Russia chooses to act contrary to international law."

The 1987 treaty put an end to a mini-arms race triggered by the Soviet Union's deployment of SS-20 nuclear missiles targeting Western European capitals.

NATO at the time responded by deploying US nuclear-tipped Pershing missiles. This led to massive pacifist demonstrations across Europe, and even heated public debates around the German pacifist slogan "better red than dead."

Several US experts and officials warn that Vladimir Putin's Russia is rebuilding its missile arsenal much in the same way as the Soviet Union.

In the United States, hawks are already arguing for US nuclear missiles to be deployed to Europe to re-establish equilibrium -- a move that, like in the 1980s, would likely result in widespread European public opposition. That could split NATO.

"I take this news as evidence that the US should build up its nuclear forces in Europe," said Tom Cotton, an Arkansas Republican.

Senator John McCain pressed Washington to "take immediate action to enhance our deterrent posture in Europe and protect our allies" due to the "most recent developments."

McCain, the Republican head of the Senate Armed Services Committee, called for "the ongoing modernization of US nuclear forces," and making sure "that NATO's nuclear deterrence forces are survivable, well-exercised and increasingly ready to counter Russian nuclear doctrine, which calls for the first use of nuclear weapons."

- Unfruitful negotiations -

Even disarmament experts worry about the new Russian missiles, and say NATO should deploy weapons to counter-balance them.

"We should make every effort to solve the issue diplomatically," said Jeffrey Lewis, a disarmament expert who founded the Arms Control Wonk blog.

"But sadly, at least so far the negotiations don't seem so productive with the Russians," Lewis told AFP.

For Lewis and other experts, Washington must maintain diplomatic channels open with Moscow while simultaneously deploying new weapons in Europe that threaten Russia.

But any such moves would have to also fall within the bounds of the INF treaty, still seen as critical to avoiding a ruinous and dangerous arms race.

"If the goal is to get the Russians to eliminate this new missile, it would be best not to" break the INF treaty, said Michael Krepon of the Washington-based Stimson Center.

The treaty already provides for "intrusive monitoring" that includes field inspections and monitoring missile production facilities.

Lewis and Krepon noted that new conventional missiles that are very fast and accurate could be deployed without violating the treaty.

The treaty does not, for example, cover missiles fired from ships or planes.

US officials did not describe the missile deployed by the Russians, but according to Lewis, it could be a ground version of the Kalibr cruise missile fired from Russian warships at rebel targets in Syria.

These missiles could easily be equipped with nuclear warheads.

Moscow, too, has accused Washington of violating the INF treaty.

It says that the missile defense system that the United States deployed in Poland and Romania could be used if necessary to launch missiles toward Russia.