A North Korean envoy says his country has nuclear weapons and is prepared to use them any time.

Hyun Hak Bong, North Korean ambassador to Britain, said in an interview with Sky News on Friday that his government would use the weapons in response to a nuclear attack by the US.

"It is not the United States that has a monopoly on nuclear weapons strikes," Hyun told Sky at the country's London embassy.

Asked if that meant North Korea, which quit the nuclear Non-Proliferation Treaty in 1993, had the capability to fire a nuclear missile now, he replied: "Any time, any time, yes."

"If the United States strike us, we should strike back. We are ready for conventional war with conventional war, we are ready for nuclear war with nuclear war. We do not want war but we are not afraid of war," Hyun said.

In a speech on March 3, Ri Su Yong, North Korean foreign minister, said his country had the power to deter an "ever-increasing nuclear threat" by the US with a pre-emptive strike if necessary.

He also denounced military exercises staged by South Korea and the US as provocative.

North Korea is thought to have a handful of crude nuclear bombs and has conducted three nuclear tests since 2006. But experts are divided on how far it has come in developing the technology needed to miniaturise warheads so they can be placed on missiles.

Kim Min-seok, spokesperson for South Korea's Defence Ministry, said on Saturday that while North Korea might have advanced its technologies for miniaturising nuclear warheads so they can be installed on missiles, Seoul does not believe they have succeeded yet.

Kim pointed out that the North has conducted only three nuclear tests so far and it is unclear how successful they were.

A spokesperson for the US State Department declined to comment "on intelligence matters" when asked if North Korea was able to fire a nuclear missile, but said the US remained "fully prepared to deter, defend against, and respond to the threat posed by North Korea".