Washington has complained for nearly two years that a ground-launched missile system deployed by Russia breaches the 1987 Intermediate Nuclear Forces Treaty (INF).

NATO defence ministers will discuss the issue at a two-day meeting in Brussels, after the alliance's July summit declaration said the Russian 9M729 missile system raised "serious concerns".

Stoltenberg said the 1987 accord signed by US president Ronald Reagan and Soviet leader Mikhail Gorbachev remained a "cornerstone of European security".

"We are very serious about our concerns and very serious when we call on Russia to comply with the INF treaty in a transparent and verifiable way," Stoltenberg said, adding that Moscow had long denied the missile even existed.

"Now they have admitted the missile exists and therefore we have called on them to answer our questions. As long as they don't answer our questions, the most plausible solution is that this is a violation of the INF treaty."

NATO leaders raised concerns about the Russian 9M729 missile system and urged Moscow to engage in dialogue to ensure the future of the INF treaty, which abolished a whole class of missiles with a range of between 500 and 5,500 kilometres.

It put an end to a mini-arms race in the 1980s triggered by the Soviet Union's deployment of SS-20 nuclear missiles targeting Western European capitals.

Moscow has repeatedly insisted it is not in breach of the treaty, but its denials have cut little ice in Western capitals.

US Defense Secretary James Mattis said in February that the Pentagon was working on new low-yield nuclear weapons to respond to the Russian move in a bid to force them back into compliance with the INF.

On Tuesday the US Ambassador to NATO Kay Bailey Hutchison set pulses racing when she said Washington was looking to "take out the missiles that are in development by Russia".

She later clarified that she was not suggesting the US would launch pre-emptive strikes on Russia, but underlining the need for the allies to find ways to counter any escalation.

A NATO official insisted "the ball on this issue is in Russia's court", saying the alliance expected "credible answers" about the missile system.

The official accused Russia of undermining longstanding international agreements aimed at reducing military tensions.

"We do have problems with the whole range of arms control and reduction treaties," the official told reporters.

Moscow has "essentially renounced" a late Cold War agreement limiting the deployment of conventional forces in Europe, the official said, and regularly fails to give observers access to military exercises.