THE United States has scrambled to reinforce its Pacific missile defences, preparing to send ground-based interceptors to Guam, as North Korea said it had authorised plans for nuclear strikes on US targets.

US Defence 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 on 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 3380km southeast of North Korea and home to 6000 American military personnel, submarines and bombers.

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

Shortly after the planned THAD deployment was announced, the North Korean military released a statement saying it had received final approval for military action against the US, 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," the statement said.

"The merciless operation of our revolutionary armed forces in this regard has been finally examined and ratified."

Yun Duk-Min, a professor at the Korea National Diplomatic Academy in Seoul, said the latest 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."

On Thursday morning, North Korea blocked access to its Kaesong joint industrial zone with South Korea for the second day running.

Pyongyang had 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 North said several hundred South Koreans currently in Kaesong - 10km inside the North Korean border - would be allowed to leave whenever they wanted.

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

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.

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.

Experts said it would take at least six months to get the reactor back up and running, after which it will be able to produce one bomb's worth of weapons-grade plutonium per year.

Originally published as US boosts defence amid N Korea warning