North Korea has moved a medium-range missile to its east coast, South Korea said Thursday as the United States strengthened its Pacific missile defences amid intensifying threats from Pyongyang.

Seoul's defence minister Kim Kwan-Jin said the missile could reach a "considerable distance" but not the US mainland.

"It could be aimed at test-firing or military drills," he told lawmakers.

North Korea, incensed at fresh UN sanctions and South Korea-US military drills, has issued a series of apocalyptic threats of nuclear war in recent weeks.

Early Thursday its 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 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.

United Nations Secretary General Ban Ki-moon told North Korea on Thursday to rein in its threats, warning that any miscalculation could lead to an extremely grave outcome.

He told a news conference in Madrid that daily security and humanitarian reports from Pyongyang were "really alarming and troubling".

"Nuclear threat is not a game, it is very serious," Ban told reporters.

"I think they have gone too far in their rhetoric and I am concerned that if by any misjudgement, by any miscalculations... this will have very serious implications," he said.

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."

Intelligence analysis quoted by South Korea's Yonhap news agency said the east coast missile was believed to be a Musudan which has an estimated range of around 3,000 kilometres or more.

Yonhap quoted sources as saying Pyongyang is likely to fire it around the middle of April, when North Koreans celebrate the birthday of their late founder Kim Il-Sung.

A provocative missile test-fired into the sea over Japan is one scenario that analysts have said the North could choose to exit the crisis with a face-saving show of force.

The European Union 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 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 conducted its third nuclear test and drew fresh UN sanctions.

Apart from its threats of nuclear attack, 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.

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

But the reopening of Yongbyon gives it a way of replenishing its bomb-making material.

North v South: military strengths on the Korean peninsula
Seoul (AFP) April 5, 2013 - North Korea's army says it has approved plans for nuclear attacks on US targets, the latest in a series of apocalyptic threats that have sent tensions soaring on the divided Korean peninsula.

Many analysts say the North's rhetoric is for internal consumption, intended to bolster the authority of young leader Kim Jong-Un over the powerful military.

But they also see the possibility that the situation could spiral out of control, with a limited border provocation triggering a wider conflict.

Here are some facts on the capabilities -- conventional and nuclear -- of the militaries on the Korean peninsula and some possible conflict scenarios.

THE MILITARY BALANCE

The North's 1.2 million-strong armed forces, the world's fifth largest, boast 4,100 tanks, 8,500 field artillery pieces and 5,100 multiple rocket launchers. It has some 620 combat aircraft.

About 600 Scud missiles are capable of hitting targets in South Korea, plus another 200-300 that could reach Japan.

The North probably has enough plutonium to make four to eight nuclear bombs, but no one knows whether it has the technology to deliver them as missile warheads.

Experts say much of its equipment is obsolete and plagued by shortages of fuel and spare parts. But conventional artillery and missiles could inflict heavy casualties and severe damage in Seoul, which lies just 50 kilometres (30 miles) south of the border, in any surprise attack.

The North is also believed to have up to 5,000 tons of chemical weapons deliverable by artillery or missile.

South Korea's 655,000-strong military is supported by US troops, based in the South since the 1950-53 war and currently numbering 28,000. The South has some 2,400 battle tanks plus 50 US tanks, 5,200 field artillery pieces and 200 multiple rocket launchers.

The South has some 460 combat aircraft plus 90 US machines, but as with most military hardware the quality is superior to the North's. At sea, it can deploy 19 principal combat vessels compared to the North's three.

Massive US reinforcements would be shipped to the peninsula if war broke out, to bolster the troops and assorted hardware including tanks and artillery already deployed on South Korean soil.

THE NUCLEAR UMBRELLA

The US army withdrew all its tactical nuclear weapons from South Korea in 1991 and the South has no such weapons. But the US guarantees a "nuclear umbrella" in case its ally comes comes under atomic attack.

WHAT MIGHT HAPPEN

Quite possibly, nothing. But the North might stage a limited border provocation like the shelling of the South's Yeonpyeong island in 2010, which killed four people.

Such an incident could quickly escalate -- the South's military has promised to hit back hard after being criticised for its perceived weak response in 2010. A South Korea-US pact signed last month provides for a joint military response even to low-level provocations.

IF WAR BROKE OUT, WHO WOULD WIN?

Despite the chilling threats, most analysts believe the North would never risk obliteration by launching a first-strike nuclear attack on US or South Korean forces.

Most North Korean troops and conventional firepower are based close to the border with the South, theoretically allowing for a swift invasion as in 1950.

Any attack could be preceded by a devastating artillery attack on military positions south of the border and on Seoul itself.

"I'm confident we would be able to stop an attack. There would still be a lot of destruction. I don't want to minimise it," General Walter Sharp, the then-US military commander in South Korea, said in a 2011 interview with the Wall Street Journal.

The London-based International Institute for Strategic Studies said that with both sides facing catastrophic losses, neither wants to fight a war for fear of the consequences.

"The danger is that war will begin out of miscalculation, misperception and escalation, rather than design," the IISS said.