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 approved 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 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 appeared 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 says some 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 and drawing fresh UN sanctions.

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 little panic.

The latest developments saw the stock market slip by 1.2 per cent Thursday, but the bourse has been remarkably resilient with the KOSPI index down barely 1.0 per cent overall since war rhetoric cranked up two weeks ago.

However the escalating crisis has triggered deep global concern, with China and Russia issuing repeated calls for restraint. The North also warned it would reopen its mothballed Yongbyon reactor — its source of weapons-grade plutonium that was closed in 2007 under an aid-for-disarmament accord.