The communist country says if the US continues its "reckless hostile policy", they are ready to use nuclear weapons any time."
North Korea has said its main nuclear complex is
fully operational and the country is ready to face US hostility with
nuclear weapons "at any time".
Its nuclear weapons are being improved "in quality and quantity", an official quoted by the state news agency KCNA said.The director of North Korea's Atomic Energy Institute said scientists had "made innovations day by day" to "guarantee the reliability of the nuclear deterrent...as required by the prevailing situation".
He added: "In the meantime, the US anachronistic hostile policy toward the DPRK that forced it to have access to the nuclear weapons has remained utterly unchanged and instead it has become all the more undisguised and vicious with the adoption of means openly seeking the downfall of the latter's social system.
"If the US and other hostile forces persistently seek their reckless hostile policy towards the DPRK and behave mischievously, the DPRK is fully ready to cope with them with nuclear weapons any time."
The words come a day after Pyongyang's warning that it is ready to launch "satellites" on rockets banned by the West.
This would mark the ruling communist party's anniversary in October and could put pressure on the US to resume talks.
The North's National Aerospace Development Administration director said satellites would be used for weather forecasts, adding that space development is "a sovereign state's legitimate right" and that the North Koreans are "fully determined to exercise that right".
He added that the world will "clearly see a series of satellites soaring into the sky at times and locations determined" by the Workers' Party.
Only last month, North Korea leader Kim Jong-Un ordered his troops to be "ready for war" with neighbour South Korea after he blamed the South for broadcasting propaganda statements across the frontier.
The North Koreans also responded by firing a shell believed to be aimed at a loudspeaker.
Urgent talks between the two appeared to have quelled tensions and a deadline imposed by the North passed without any further attacks.
The Korean War ended in 1953 with an armistice, not a peace treaty, leaving the peninsula in a technical state of war.
The US has about 28,500 troops in South Korea as deterrence against potential aggression from North Korea.