North Korea has pledged to strengthen its nuclear weapons programme, a day after announcing it is in a "state of war" with South Korea, as the region remains tense amid a military build-up by both North and South Korea.
A meeting on Sunday of the central committee of the ruling Workers' Party, headed 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.
Tensions have risen sharply on the peninsula since the UN tightened sanctions in response to the North's nuclear and missile tests, and the US and South Korea carried out military drills near the border with the North .
On Saturday, the North warned South Korea and its close ally US that any provocation would swiftly escalate into an all-out nuclear conflict.
"As of now, inter-Korea relations enter a state of war and all matters between the two Koreas will be handled according to wartime protocol," the North said in a statement carried by the KCNA.
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.
The party summit also decided to develop a light water reactor as part of a civilian nuclear power industry to ease electricity shortages, KCNA said.
Technically at war
The North also threatened on Saturday to shut down a joint industrial complex with South Korea, but an official from the South said the complex, which lies inside North Korea, was operating normally on Sunday.
The two Koreas have always technically remained at war because the 1950-53 Korean War concluded with an armistice rather than a peace treaty.
The South Korean government says that its defence drills with the United States were routine, but the North views those drills as rehearsals for an invasion.
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On Friday, North Korean leader Kim Jong-un warned his forces were ready "to settle accounts with the US" after two American B-2 bombers flew a training mission in South Korea.
The White House said it took the warning from North Korea that it was in a state of war with the South "seriously", but said Pyongyang's threats were following a familiar pattern.
"We've seen reports of a new and unconstructive statement from North Korea. We take these threats seriously and remain in close contact with our South Korean allies," said Caitlin Hayden, spokeswoman for the US National Security Council.
The North had announced earlier this month that it was tearing up the armistice and other bilateral peace pacts signed with the South in protest against South Korea-US joint military exercises.
The armistice was approved by the UN General Assembly, and both the UN and South Korea have repudiated the North's unilateral withdrawal.
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.
It formally abandoned six-party talks in in April 2009 offering it economic and security benefits in return for denuclearisation.