North Korea is only months away from obtaining the capability to hit U.S. territory with a nuclear weapon and must be disarmed, a U.S. envoy said on Tuesday, dismissing Pyonyang's diplomatic thaw with South Korea as a "charm offensive" that fooled no one.

In a diplomatic showdown at a U.N.-sponsored Conference on Disarmament, North Korea responded by blaming Washington for escalating confrontation, saying it was deploying nuclear assets including aircraft carriers near the divided peninsula and was considering a pre-emptive strike against Pyongyang.

"North Korea has accelerated its provocative pursuit of nuclear weapons and missile capabilities, and expressed explicit threats to use nuclear weapons against the United States and its allies in the region," U.S. disarmament ambassador Robert Wood told the Geneva forum.

"North Korean officials insist that they will not give up nuclear weapons, and North Korea may now be only months away from the capability to strike the United States with nuclear-armed ballistic missiles," he said.

A new U.S. nuclear policy review outlined last week "reaffirms that North Koreas illicit nuclear program must be completely, verifiably, and irreversibly eliminated, resulting in a Korean Peninsula free of nuclear weapons," he said.

Asked later what the basis was for the assessment that North Korea would soon be able to hit the United States with a nuclear weapon, he said he had "no new information to share."

North Korea tested its first intercontinental ballistic missile, the Hwasong-14, twice last July. In November it tested the Hwasong-15, believed to be capable of reaching the continental United States. It is not yet believed to have the capability to mount a nuclear warhead on a ballistic missile.

North Korea is under tightening U.N. Security Council sanctions for its banned nuclear and ballistic missile programmes. But recent weeks have seen a thaw with South Korea, after Pyongyang agreed to send athletes to compete in the Olympic Games opening on Feb. 9 in the south.