SEOUL, March 31 –  North Korea vowed Sunday to strengthen its nuclear weapons, a day after announcing it is in a "state of war" with South Korea, and said it would never trade its atomic deterrent for aid.

Tensions have risen sharply on the peninsula since the United Nations tightened sanctions in response to the North's nuclear and missile tests, and the United States and South Korea launched military drills south of the border.

On Saturday the North declared it was in a "state of war" with the South and warned Seoul and Washington that any provocation would swiftly escalate into an all-out nuclear conflict.

A meeting Sunday of the central committee of the ruling Workers' Party, guided by leader Kim Jong-Un, decided that the country's possession of nuclear weapons "should be fixed by law", the official KCNA news agency reported without elaborating.

The nuclear armed forces "should be expanded and beefed up qualitatively and quantitatively until the denuclearisation of the world is realised", it added.

Members also decided to develop a light water reactor as part of a civilian nuclear power industry to ease electricity shortages, KCNA said.

The North in 2010 disclosed the existence of a uranium enrichment facility and light water reactor, purportedly to generate power. Experts said it could easily be reconfigured to make fuel for nuclear weapons.

The North in April 2009 formally abandoned six-party talks offering it economic and security benefits in return for denuclearisation.

On Sunday it reiterated that its atomic weapons are not a bargaining chip.

"They are a treasure of a reunified country which can never be traded with billions of dollars," KCNA quoted the central committee members as saying.