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North Korea warned last night it had ratified a “merciless attack” against the United States in a war which could start “today or tomorrow”.

As leader Kim Jong-un’s aggressive rhetoric hit a new level, the army threatened attacks by a “smaller, lighter and diversified” nuclear force.

"The moment of explosion is approaching fast," the North Korean military said, warning that war could break out "today or tomorrow".

A spokesman for the General Staff of the Korean People’s Army added: “We formally inform the White House and Pentagon that the ever-escalating US hostile policy toward us and its reckless nuclear threat will be smashed.”

In response, President Obama ordered a ballistic missile defence system to be moved to the nearest US military base on the Pacific island of Guam, 2,000 miles from North Korea’s capital Pyongyang.

As the threat grew, PM David Cameron warned against any move to abandon Britain’s Trident nuclear deterrent.

He said: “Does anyone seriously argue that it would be wise for Britain, faced with this evolving threat today, to surrender our deterrent?”

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US Defence Secretary Chuck Hagel said of North Korea: “Some of their actions present a real danger and threat to the interests of our allies, starting with South Korea and Japan.

“There have been threats the North Koreans have levelled at our bases in Guam and Hawaii. We are doing all we can to defuse that.

"I hope the North will ratchet this very dangerous rhetoric down,"

He added that there is a way to peace but only if Kim Jong Un decides to be "a responsible member of the world community."

China has expressed “serious concern” with their Deputy Foreign Minister Zhang Yesui meeting with officials from North and South Korea and the US.

Foreign Ministry spokesman Hong Lei said: “All sides must remain calm and exercise restraint.”

Yesterday North Korea stopped workers in the South crossing the border to a jointly run industrial zone for the first time in four years.

More than 800 South Koreans who had stayed overnight at the Kaesong complex were being allowed to return home but new workers were not being allowed across the border.

Kaesong, a major source of income for the impoverished, communist North, is home to 124 South Korean companies that employ 53,000 North Korean workers in a cross-border, heavily fortified joint enterprise.

Permission is granted on a daily basis for South Korean workers to cross into the complex, situated in the North.