The United States has scrambled to reinforce its Pacific missile defences, preparing to send ground-based interceptors to Guam, as North Korea said Thursday it had authorised plans for 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 to its allies South Korea and Japan.

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

The Pentagon said it would send ground-based THAAD missile-interceptor batteries to protect military 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, submarines and bombers.

They would complement two Aegis anti-missile destroyers already dispatched to the region.

Shortly after the THAAD announcement, 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 ongoing 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, reports Thursday said it appears to have moved a medium-range missile capable of hitting targets in South Korea and Japan to its east coast.

"We are closely monitoring whether the North moved it with a view to actual launch or just as a show of force against the US," Yonhap news agency quoted a South Korean official as saying.

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.

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 the Kaesong complex, 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 (CPRK) said.

The North has said that the more than 800 South Koreans currently in Kaesong -- 10 kilometres (six miles) inside the North Korean border -- can leave whenever they want but many have chosen to stay to keep the factories running.

North Korea 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.

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.

Subsequent UN sanctions and joint South Korea-US military drills triggered weeks of near-daily threats from Pyongyang, ranging from artillery strikes to nuclear armageddon.

The escalating crisis has triggered global concern, with China and Russia issuing repeated calls for restraint and UN Chief Ban Ki-moon warning that the situation had "gone too far" and risked spiralling out of control.

This week, the North warned it would reopen its mothballed Yongbyon reactor -- its source of weapons-grade plutonium. It was closed in July 2007 under a six-nation aid-for-disarmament accord.

The US-Korea Institute at John Hopkins University said Wednesday that a satellite photograph seen on March 27 appeared to show construction work around the reactor was already under way.

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