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 atomic 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 DP

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 the satellite 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.