Fears are growing of a new arms race in Europe after Washington started the process of exiting the Intermediate-Range Nuclear Forces (INF) Treaty claiming that Russia violated the pact with a new missile system.

NATO Secretary General Jens Stoltenberg said the alliance would beef up its defences but insisted this did not mean "mirroring" any Russian build-up of missiles.

The fate of the INF treaty, signed by the US and the Soviet Union in 1987 to ban ground-launched mid-range missiles, will be high on the agenda as NATO defence ministers meet in Brussels on Wednesday.

Western capitals want Russia to return to compliance with the treaty by abandoning its new 9M729 missile system.

"We are both urging Russia to come back in compliance but at the same time we are planning for a world without the INF treaty and with more Russian missiles," Stoltenberg said.

"We don't have to mirror what Russia does but we need to make sure we have effective deterrence and defence."

Stoltenberg repeated warnings that the new Russian missiles made nuclear conflict more likely because they are mobile, hard to detect, and give little warning time.

The US Ambassador to NATO, Kay Bailey Hutchison, said the Pentagon has already started looking at how best to defend against the new missiles.

"America felt it was time for us to have a defence and not be left without a defence with Russia having missiles that were in violation," she said.

"What is going to happen going forward? First of all, the defence we would be working on is conventional, not nuclear."

The INF treaty banned ground-launched missiles with a range of 500 to 5,500 kilometres (310 to 3,400 miles), ending a dangerous build-up of warheads on mainland Europe.

Russian President Vladimir Putin responded to the US pullout by saying Moscow would also leave the treaty, and his defence minister announced plans for new missiles -- prompting Trump to vow to outspend Moscow.

While pointing the finger at each other, both Washington and the Kremlin have voiced concern that the bilateral INF treaty does nothing to constrain China, whose rapidly growing military relies on medium-range missiles as a core part of its defence strategy.