Washington, May 21 (IANS): The US on Wednesday cast doubt on Pyongyang's claimed capacity to miniaturise and diversify its stockpiled nuclear weapons.

"Regarding that specific claim of miniaturisation, we do not think they have that capacity," State Department spokesperson Marie Harf told reporters, Xinhua reported.

North Korea said earlier in the day that it has entered the phase of miniaturisation and diversification of its nuclear weapons for quite some time, with the successful test-firing of a strategic ballistic missile from a submarine on May 8.

The Asian country also blasted the US, Japan and South Korea for their claim that Pyongyang's test-firing of the submarine-launched ballistic missile is a threat and provocation, saying it is North Korea's right to beef up its defence ability.

"But we do know they're working on developing a number of long-range missiles, including intercontinental ballistic missiles," Harf said.

"That could eventually threaten our allies, our partners...So that's obviously something we're very concerned about in terms of that capability. We just don't think they have it," she added.