North Korea Vows to Strengthen Nuclear Arsenal

North Korea has vowed to develop and launch more advanced satellites and says the country's nuclear weapons are non-negotiable and will not be traded even for "billions of dollars" in economic assistance.

The official KCNA news agency reported Sunday that a plenary meeting of the North Korean Workers' Party Central Committee, chaired by leader Kim Jong Un, called for building both a stronger economy and nuclear arsenal.

It said the North's nuclear armed forces are not a political bargaining chip, and that they should be "expanded and beefed up...until the denuclearization of the world is realized."

The meeting comes a day before the country's rubber stamp parliament is to meet April 1 amid increased tensions with South Korea and the United States.

In a related development, sources told VOA that a group of U.S. F-22 stealth fighter jets were flown Sunday from Kadena Air Base in Japan to Osan Air Base, south of Seoul, to join the current joint exercises in South Korea.

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Last week, a pair of stealth B-2 bombers were dispatched from the United States to drop inert munitions on a South Korean island range.

The mission, following those by B-52 bombers earlier in the month, was seen as sending a message to both Seoul and Pyongyang. It was a dramatic reassurance to the South that it is protected under the United States nuclear umbrella -- and a warning to the North of U.S. forces' capability to strike precisely and quickly from a long distance, should war erupt.

On Saturday, the North said it had entered a "state of war" with South Korea and warned any provocation by Seoul and Washington would escalate into nuclear confrontation. Pyongyang said all issues between the two Koreas would be handled according to "wartime regulations."

In response, South Korea's defense ministry urged its northern neighbor to stop making threats. The ministry also said the South's forces will completely punish the North if there is a provocation.

VOA Seoul Bureau Chief Steve Herman said the Korean peninsula "has not seen this level of tension since late 2010, when the North bombed the South’s Yeonpyeong Island,” killing four. Pyongyang justified the attack by pointing to South Korea’s shelling of disputed waters during a military exercise.

South Korean defense sources said Friday that vehicles and personnel were seen moving toward a North Korean missile launch test site, but that no activity had been detected.

The White House said it takes Pyongyang's latest statement seriously. National Security Council spokeswoman Caitlin Hayden said the North has a long history of bellicose rhetoric, and that Saturday's threat follows a familiar pattern.

North Korea also threatened to shut down a joint industrial complex with the South on Saturday. A North Korean spokesman said the Kaesong industrial complex just north of the line (Demilitarized Zone) separating the two countries will close if Seoul undermines the North's dignity.

North Korea has directed a series of vitriolic comments at the U.S. and South Korea in recent weeks. Pyongyang has threatened to turn Seoul into a "sea of fire" and has warned of firing rockets at U.S. military bases in Guam, Hawaii and Japan.

Analysts say the North is not yet capable of mounting an operational nuclear warhead on a missile. But many of its neighbors are worried they may be easier targets for Pyongyang's conventional weapons.