"Restraint and responsibility in the nuclear domain is crucial to global security," an official from the US-led military alliance said, on condition of anonymity.

The official said Russia and China were both ramping up their nuclear capabilities and that NATO "will continue to take steps necessary" to ensure its own defences.

"Russia's irresponsible nuclear rhetoric and coercive nuclear signalling demonstrate a posture of strategic intimidation," the official said.

Meanwhile, "China continues to rapidly expand and diversify its nuclear arsenal with more warheads and a larger number of sophisticated delivery systems," they added.

The Kremlin on Thursday said it regretted the end of the last treaty governing the nuclear arsenals of Russia and the United States and called it a negative step.

Washington has said any new nuclear agreement would have to include China, but international efforts to encourage Beijing to join fresh talks have so far failed.