North Korea says can fire nuclear missile at 'any time'

North Korea has the ability to fire a nuclear weapon and would use a nuclear missile in retaliation if it is attacked, the country's ambassador to Britain told Sky News on Friday.

"It is not the United States that has a monopoly on nuclear weapons strikes," Ambassador Hyun Hak-bong told Sky at the isolated Asian 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."

He added: "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."

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

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

The United States has said it is seriously concerned about North Korea's nuclear work, which it says breaches international agreements.

On Wednesday, the US Missile Defense Agency said it was carrying out increasingly realistic tests of missile defences, citing the growing missile threats from North Korea and Iran.

MDA Director Navy Vice Admiral James Syring said North Korea has fielded hundreds of missiles that could reach US forces based in South Korea and Japan.

Syring told the defense subcommittee for the US Senate Appropriations Committee that both North Korea and Iran could achieve the ability to launch an intercontinental ballistic missile as early as this year.