The United States has scrambled to reinforce its Pacific missile defences as North Korea pushed more global alarm buttons Thursday by announcing it had authorised plans for possible nuclear strikes on US targets.

US Defense Secretary Chuck Hagel said Pyongyang's increasingly bellicose threats combined with its military capabilities represented a "real and clear danger" to the United States and its allies South Korea and Japan.

The Pentagon said it would send ground-based THAAD missile-interceptor batteries to protect bases on Guam, a US territory some 3,380 kilometres (2,100 miles) southeast of North Korea and home to 6,000 American military personnel.

"They have nuclear capacity now, they have missile delivery capacity now," Hagel said Wednesday. "We take those threats seriously."

Shortly afterwards, the North Korean military said it had received final approval for military action against the United States, possibly involving nuclear weapons.

"The moment of explosion is approaching fast," the Korean People's Army general staff said, responding to what it called the provocative US use of nuclear-capable B-52 and B-2 stealth bombers in war games with South Korea.

The US aggression would be "smashed by... cutting-edge smaller, lighter and diversified nuclear strike means," it said in a statement.

While few of the North's threats have been matched with action, South Korea said it appeared to have moved a medium-range missile to its east coast.

"It could be aimed at test-firing or military drills," South Korean Defense Minister Kim Kwan-Jin told lawmakers.

A provocative missile test-fired into the sea over Japan is one scenario that analysts have said the North could opt for as a relatively low-risk way of exiting the crisis with a face-saving show of force.

The new nuclear threats drew fresh concern led by UN chief Ban Ki-moon, who said on a visit to Monaco that he was "deeply concerned and troubled" over the escalating rhetoric.

"At this time, I think all the parties concerned in the Korean peninsula, in particular the Chinese government, can play a very important role to calm down the situation," said Ban, a former South Korean foreign minister.

The European Union also called on Pyongyang to stop stoking tensions and re-engage with the international community.

Russia's foreign ministry termed the North's neglect of UN resolutions as "categorically unacceptable".

Yun Duk-Min, a professor at the Korea National Diplomatic Academy in Seoul, said the latest nuclear threat was similar to one issued a month ago, but with the added weight of "approval" -- presumably by North Korean leader Kim Jong-Un.

"The problem is whether Kim, who is still young and inexperienced, knows how to handle this escalation," Yun said. "Where does it end? That's the worrying question."

North Korea blocked access to its Kaesong joint industrial zone with South Korea Thursday for the second day running, and threatened to pull out its 53,000 workers in a furious reaction to the South's airing of a "military" contingency plan to protect its own workers there.

Pyongyang informed Seoul on Wednesday it was stopping the daily movement of South Koreans to Kaesong, which lies 10 kilometres (six miles) inside the North and is the last real surviving point of contact between the two countries.

"The full closure of the complex is set to become a reality," a spokesman for the North's Committee for Peaceful Reunification of Korea said.

The North says the South Koreans currently in Kaesong can leave whenever they want. About 200 departed Thursday but some 600 remain to keep the factories running.

Tensions have soared on the Korean peninsula since December, when the North test-launched a long-range rocket. In February, it upped the ante once again by conducting its third nuclear test and drawing fresh UN sanctions.

It previously threatened a "pre-emptive" nuclear strike against the United States in early March, and last week its supreme army command ordered strategic rocket units to combat status.

Most experts think it is not yet capable of mounting a nuclear device on a ballistic missile capable of striking US bases or territory.

The North also warned this week it would reopen its mothballed Yongbyon reactor -- its source of weapons-grade plutonium that was closed in 2007 under an aid-for-disarmament accord.

South Koreans have become accustomed to the North's threats and provocations over the years, and even with tensions as heightened as they are now, there is some public anxiety but no panic.

The latest developments saw the stock market slip by 1.2 percent Thursday in its biggest one-day loss in four months. But the bourse has been remarkably resilient since war rhetoric cranked up in earnest two weeks ago.

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