The White House has told North Korea to stop making threats after the isolated state dramatically upped its warlike rhetoric, saying it had authorised plans for nuclear strikes on US targets.

"The moment of explosion is approaching fast," the state news agency KCNA reported the North Korean military as saying, warning that war could break out "today or tomorrow".

"We formally inform the White House and Pentagon that the ever-escalating US hostile policy toward the DPRK (North Korea) ... [will be smashed by the] cutting-edge smaller, lighter and diversified nuclear strike means of the DPR."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 2 minutes 45 seconds 2 m 45 s North Korea 'front and centre' in talks Download 1.3 MB

Pyongyang's latest pronouncement came as Washington scrambled to reinforce its Pacific missile defences, preparing to send ground-based interceptors to Guam and dispatching two Aegis class destroyers to the region.

Military strength: North v South North Korea South Korea Active troops 1.2 million 655,000 Reserves 5-7.7 million 3 million Tanks 4,100 2,400 Armoured personnel carriers 2,500 2,600 Rocket launchers 5,100 200 Mortars 7,500 6,000 Air defence guns 11,000 300 Combat aircraft 820 (620 serviceable) 460 Helicopters 300 680 Combat vessels 3 19 Submarines 70 23

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

It has since threatened to pull its 53,000 workers from the Kaesong industrial zone close the complex.

"We've seen today's statement by North Korea, again making unhelpful and unconstructive threats," US National Security Council spokeswoman Caitlin Hayden said.

"It is yet another offering in a long line of provocative statements that only serve to further isolate North Korea from the rest of the international community and undermine its goal of economic development.

"North Korea should stop its provocative threats and instead concentrate on abiding by its international obligations."

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

But while Pyongyang has successfully carried out test nuclear detonations, most experts think it is not yet capable of mounting a device on a ballistic missile capable of striking US bases or territory.

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

Earlier, US defence secretary Chuck Hagel said Pyongyang represented a "real and clear danger" to the US and allies South Korea and Japan.

"They have nuclear capacity now, they have missile delivery capacity now," Mr Hagel said after a strategy speech at the National Defence 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.

"I hope the North will ratchet its very dangerous rhetoric down."

'Ready to defend'

The THAAD is a truck-mounted system that can pinpoint an enemy missile launch and launch an interceptor to bring it down. ( AFP, file photo )

The Pentagon said it would send ground-based THAAD (Terminal High Altitude Area Defence) interceptor batteries to protect US bases on the island of Guam, complementing two Aegis anti-missile destroyers already dispatched to the region.

North Korea has singled out US military bases in Guam and Hawaii among its potential targets in threats in recent weeks.

"The United States remains vigilant in the face of North Korean provocations and stands ready to defend US territory, our allies, and our national interests," a Pentagon spokeswoman said.



Mark Borthwick, a director of the US Asia Pacific Council in Washington, has told Radio Australia's Pacific Beat program that the deployment sends a message to America's allies that it is taking the situation seriously.

"It means at a high altitude it can cover a very broad area of defence, and the capacity therefore amounts to a good deal more than Guam," he said.

"Similarly the destroyer capacity could also cover a broad range, so really the idea is to demonstrate... Guam may be a staging point but we are looking out for a broader region."

The THAAD is a truck-mounted system that can pinpoint an enemy missile launch, track the projectile and launch an interceptor to bring it down.

Guam is a US island territory 3,380 kilometres south-east of North Korea in the Pacific and is home to 6,000 American military personnel, as well as bases for submarines and strategic bombers.

Meanwhile, North Korea has blocked access to its joint industrial zone with South Korea for a second consecutive day, an AFP journalist near the border between the two countries said.

About 40 vehicles were waiting to cross the border to go to the Seoul-invested Kaesong complex but turned around after a loudspeaker announcement that crossings to the North were prohibited, he said.

A total of 526 South Koreans and 421 vehicles had planned to cross the border into North Korea on Thursday, the unification ministry in Seoul said.

Preparing to defend itself... a United States Air Force F-16 fighter jet takes off at a base in Osan, south of Seoul in South Korea. ( Reuters: Lee Jae-Won )

ABC/wires