The US has threatened to respond to what it believes is Russia's covert development of a new missile system capable of launching quick nuclear strikes against Nato countries.

Washington's ambassador to Nato said the US remained committed to a diplomatic solution, but would "match its capabilities" if the 9M729 system becomes operational.

America believes Russia is developing the ground-launched missiles in breach of a Cold War treaty that could allow Moscow to launch a nuclear attack on Europe at short notice, but the Kremlin has consistently denied any such violation.

The 1987 Intermediate-range Nuclear Forces Treaty (INF) bans an entire class of weapons - all land-based cruise missiles with a range between 500-5,500km. Nato says the Russian system fits into that category.

Kay Bailey Hutchison initially appeared to suggest the US might consider military action if Russia continued its development of the system.

"It is time now for Russia to come to the table and stop the violations," Kay Bailey Hutchison told reporters in Brussels, on the eve of a meeting between US defense secretary James Mattis and his Nato counterparts.

She said if the system "became capable of delivering", the US "would then be looking at the capability to take out a missile that could hit any of our countries in Europe and hit America".

US ambassador to Nato Kay Bailey Hutchison briefs media (Reuters)

"Countermeasures (by the US) would be to take out the missiles that are in development by Russia in violation of the treaty," she added. "They are on notice."

But Ms Hutchison later clarified in a tweet she was not talking about a preemptive strike against Russia.

"My point: Russia needs to return to INF Treaty compliance or we will need to match its capabilities to protect US and Nato interests. The current situation, with Russia in blatant violation, is untenable."

The INF bans medium-range missiles capable of hitting Europe or Alaska.

Royal Air Force fighter jets scrambled to intercept Russian patrols in the Black Sea

In Washington, state department spokesperson Heather Nauert said the US had for some time argued that Moscow was not in compliance with the treaty.

"What Ambassador Hutchison was talking about was improving overall defence and deterrence posture," Ms Nauert told a news conference. "The United States is committed to upholding its arms control obligations and expects Russia to do the very same thing," she said, adding the US was discussing the issue with its allies.

The Russian foreign ministry said such statements were dangerous, and that it would provide detailed answers to Washington.

"It seems that people who make such statements do not realise the level of their responsibility and the danger of aggressive rhetoric," TASS news agency quoted spokesperson Maria Zakharova as saying.

In the past, the ministry has said it is ready for talks with the US to try to preserve the treaty and would comply with its obligations if the US did.

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But that same year, the US state department report found Russia had violated obligations "not to possess, produce, or flight-test" a ground-launched cruise missile with a range capability of 500 km to 5,500km, "or to possess or produce launchers of such missiles".

The US accusations are likely to further strain relations between Moscow and the West that are at a low over Russia's 2014 seizure of Crimea, its bombing campaign in Syria and accusations of meddling in Western elections.

"We have been trying to send a message to Russia for several years that we know they are violating the treaty; we have shown Russia the evidence that we have that they are violating the treaty," Ms Hutchison said.

"We are laying down the markers so that our allies will help us bring Russia to the table," she added.

US secretary of defense Jim Mattis said he would discuss the issue with his Nato counterparts at a scheduled two-day meeting in Brussels from Wednesday.

"I cannot forecast where it will go, it is a decision for the president, but I can tell you that both on Capitol Hill and in state department, there is a lot of concern about this situation and I'll return with the advice of our allies and engage in that discussion to determine the way ahead," he told reporters in Paris.