North Korea said it has ratified plans for nuclear strike against the United States, warning that the war could break out "today or tomorrow".

North Korea said it has ratified plans for nuclear strike against the United States, warning that the war could break out "today or tomorrow".

Tension was also high on the North's heavily fortified border with South Korea, after Pyongyang barred South Koreans from entering a Seoul-funded joint industrial park on its side of the frontier.

In a statement published by the state KCNA news agency, the Korean People's Army general staff warned Washington that US threats would be "smashed by... cutting-edge smaller, lighter and diversified nuclear strike means".

"The merciless operation of our revolutionary armed forces in this regard has been finally examined and ratified," the statement said.

Last month, North Korea threatened a "pre-emptive" nuclear strike against the United States, and last week its supreme army command ordered strategic rocket units to combat status.

Mounting tension in the region could however trigger incidents on the tense and heavily militarized border between North and South Korea.

Few hours later on Thursday, Pyongyang also threatened to pull its 53,000 workers from the Kaesong joint industrial zone with South Korea and close the complex.

“If the South Korean puppets and conservative news media keep badmouthing (us), we will order all our workers to pull out from Kaesong,” a spokesman for the North's Committee for Peaceful Reunification of Korea (CPRK)was quoted as saying by KCNA.

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On the other hand, UN Chief Ban Ki-moon said he was deeply concerned by tensions on the Korean Peninsula, urging Pyongyang lift restrictions on S. Korea workers.

For his part, US Defense Secretary Chuck Hagel earlier said Pyongyang represented a "real and clear danger" to the United States and to its allies South Korea and Japan.

"They have nuclear capacity now, they have missile delivery capacity now," Hagel said after a strategy speech at the National Defense University. "We take those threats seriously, we have to take those threats seriously.

"We are doing everything we can, working with the Chinese and others, to defuse that situation on the peninsula."