North Korea on Friday vowed to expand and modernise its nuclear deterrent "beyond imagination" unless the United States changes its policy towards the communist nation.

The North said in a foreign ministry memorandum that it had developed nuclear weapons to defend itself against an alleged US atomic threat.

The memorandum, published by the official news agency, said the North had no choice but to prolong the possession of nuclear weapons "unless the United States makes a right choice".

"Our nuclear deterrent will be modernised and expanded far beyond US imagination," it said.

The North has been developing nuclear weapons for decades. Its official position has been that it needs them for self-defence against a US nuclear threat, but that it is willing in principle to scrap the atomic weaponry.

Under a September 2005 deal reached during six-nation negotiations, Pyongyang agreed to dismantle its nuclear programme in return for economic and diplomatic benefits and security guarantees.

But six-party talks on implementing the deal stalled in December 2008. The North has staged two nuclear tests -- in 2006 and 2009.

The International Atomic Energy Agency said Thursday that the North's nuclear programme is "a matter of serious concern".

North Korea's statements about uranium enrichment activities and the construction of a light-water reactor "continue to be deeply troubling", it said.

Pyongyang said it was building a uranium enrichment plant to produce low-enriched fuel for the new reactor to meet the nation's energy requirements.