UPDATE: AN ADVANCED ballistic missile defence system will be deployed in the Pacific in a direct response to increased threats from North Korea, the Pentagon said today.
The Terminal High Altitude Area Defence System (Thaad) will be stationed on the Pacific island of Guam within weeks, in what appears to be a direct response to heightened rhetoric from North Korea.
Shortly before the Pentagon announcement, the North Korean Army said it had ratified a "merciless" attack against the US - potentially involving a "cutting-edge" nuclear strike.
"The moment of explosion is approaching fast," warned the military in a statement on the state news agency KCNA. With war ready to break out "today or tomorrow," Pyongyang said the US had "better ponder over the prevailing grave situation".
US authorities claimed the deployment of the nuclear missile defence system would "strengthen our regional defence posture against the North Korean regional ballistic missile threat".
Both developments came after North Korea yesterday barred South Korean workers from entering the Kaesong joint industrial zone, north of their demilitarised border.
The move, which will only deepen tensions on the peninsula, came 24 hours after the North announced it was restarting the mothballed plutonium reactor at its Yongbyon facility, thereby signalling its intention to step up its nuclear weapons programme.
The interruption at Kaesong is seen by some experts as an especially ominous development, given that - with the exception of three days in 2009 - the complex had remained open through the string of previous crises since it opened in 2004 and is a rare enduring instance of co-operation between the two Koreas.
The closure "is something that was less expected, and is less directly in North Korea's interests," Patrick Cronin, a senior analyst with the Centre for a New American Security in Washington, said. "Is this a short-term demonstration of dissatisfaction with US-South Korean policy, or a portent of something more drastic at Kaesong?"
For the South, the world's 15th largest economy, Kaesong is of relatively minor significance, producing annually some $470m worth of goods which are shipped back to South Korea and then exported.
But for the North it is a precious source of hard currency and jobs - but one that for now at least seems in jeopardy.
The move is thus being generally seen as yet another act of brinkmanship in the North's confrontation with South Korea and its protector, the US.
As Pyongyang has stepped up its provocations, so the US and North Korea have continued, more ostentatiously than ever, the military exercises that they conduct each spring.
After despatching nuclear-capable B-2 stealth bombers from their base in the US to take part, Washington has positioned a second missile-defence destroyer off the Korean peninsula - in reaction to Pyongyang's recent threats to target US bases in South Korea, where 28,500 American troops are stationed, and in the Pacific.
"Let me be perfectly clear," Secretary of State John Kerry said this week after talks with his South Korean counterpart Yun Byung-se, "the United States will defend and protect ourselves and our treaty ally, the Republic of Korea [South Korea]."
The Defence Secretary Chuck Hagel, went further yesterday, claiming North Korea's actions represented a "real and clear danger" to the US and its allies.
Although Pyongyang's current rhetoric is of a violence without precedent, some US analysts still believe the North is playing a familiar game - doing everything in its power to secure concessions from the West.
They would come both in the shape of economic assistance and diplomatically, in a resumption of the long-suspended six-nation talks over its nuclear programme.
So far though the US has given not the slightest public indication it is ready to go along.
Kim Jong-Un threatens 'all-out nuclear war'
NORTH Korea stepped up its pugnacious rhetoric still further yesterday by warning Seoul that the Korean Peninsula was entering "a state of war".
The statement comes in the wake of some of Pyongyang's strongest ever threats, including a willingness to launch missiles against US bases.
Analysts say a full-scale conflict is extremely unlikely, noting that the Korean Peninsula has remained in a technical state of war for 60 years.
But the North's increasingly bellicose words - the most intemperate uttered in many a decade, including threats to launch a nuclear strike - have raised worries that a misjudgement by either side in how to address the warnings could lead to a clash.
The situation is now not unlike the second act of a rather bad Cold War film. In North Korea, the young leader is photographed surrounded by elderly military officers in their faintly comic hats, while, in the background, maps show New York, Washington, and Austin, Texas, as missile targets.
Meanwhile, in the southern half of the peninsula, the US and its ally continue to act out their annual military exercises with state-of-the-art bombers firing blanks. These manoeuvres will go on until the end of April.
North Korea's threats are seen as efforts to provoke the new government in Seoul, led by President Park Geun-hye, to change its policies towards Pyongyang, and also to win diplomatic talks with Washington so it could get more aid.
The aggressive posturing may also be part of building up the military credentials of Kim Jong-Un, who has been in post for only 16 months, providing some patriotic cheerleading for domestic consumption, and also showing anger over UN sanctions imposed over a recent underground nuclear test.
Some of it is also a direct response to the military exercises in the South.
On Thursday, for instance, US officials revealed that two B-2 stealth bombers dropped dummy munitions on front lines as part of drills with South Korean troops.
Hours later, Kim ordered his generals to put rockets on stand-by and threatened to strike American targets if provoked.
North Korea said in a statement yesterday that it would deal with South Korea according to "wartime regulations" and would retaliate against any provocations by the US and South Korea without notice.
"Now that the revolutionary armed forces of the DPRK [Democratic People's Republic of Korea] have entered into an actual military action, the inter-Korean relations have naturally entered the state of war," said the statement, which was carried by Pyongyang's official Korean Central News Agency.
The statement added: "Provocations will not be limited to a local war, but develop into an all-out war, a nuclear war."
In Washington, Caitlin Hayden, spokeswoman for the White House National Security Council, said: "We've seen reports of a new and unconstructive statement from North Korea. We take these threats seriously and remain in close contact with our South Korean allies.
"We remain fully prepared and capable of defending and protecting the United States and our allies. We continue to take additional measures against the North Korean threat, including our plan to increase the US ground-based interceptors and early-warning and tracking radar, and the recent signing of a South Korean-US counter-provocation plan."
Saturday's belligerence also included a threat by the North to shut down a factory complex that is the last major symbol of inter-Korean co-operation.
The Kaesong industrial park, which is run with North Korean labour and South Korean know-how, has been operating normally, despite Pyongyang shutting down a communications channel typically used to co-ordinate travel by South Korean workers to and from the park, just across the border in North Korea.
An identified spokesman for the North's office controlling Kaesong said yesterday that it would close the factory park if South Korea continued to "undermine its dignity". Pyongyang had felt slighted by media reports suggesting the factory only remained open because it was a source of hard currency for the impoverished North.
Dozens of South Korean firms run factories in the border town of Kaesong. Using North Korea's cheap, efficient labour, the Kaesong complex produced goods worth $470m in 2012. North Korea has, however, previously made such threats with regard to Kaesong without acting on them.
Across North Korea, soldiers are reported to be gearing up for battle and shrouding their vehicles with camouflage netting. Newly painted signboards and posters call for "death to the US imperialists" and urge the people to fight with "arms, not words".
But even as Kim Jong-un issues midnight battle cries to his generals to ready their rockets, he and his million-man army know full well that a successful missile strike on US targets would be suicide for the out-powered North Korean regime.
Some 28,500 American troops are still stationed in South Korea, and 50,000 more are in nearby Japan.
North Korea cites what it perceives as a US military threat as a key reason behind its need to build nuclear weapons, and has poured a huge chunk of its small national budget into defence, science and technology.
In December, scientists launched a satellite into space on the back of a long-range rocket using technology that could easily be converted for missiles; in February, they tested an underground nuclear device as part of a mission to build a bomb they can load on a missile capable of reaching the US.
Most observers see North Korea's bombs and missiles as more of an expensive, dangerous safety blanket than real firepower. They are the only real playing card North Korea has left.
But the reach of Kim Jong-un's missiles is far more limited than the optimistic maps on his office walls suppose. Narushige Michishita, director of security and international studies at Japan's National Graduate Institute for Policy Studies, isn't convinced that North Korea is capable of attacking Guam, Hawaii or the US mainland.
He says Pyongyang hasn't successfully tested an intercontinental ballistic missile. But its medium-range Rodong missiles, with a range of about 800 miles (1,300km), are "operational and credible" and could reach US bases in Japan, he says.
More likely, however, is a smaller-scale incident, perhaps off the western coast of the Korean Peninsula, that would not provoke the Americans to unleash their considerable firepower.
For years, the waters off the west coast have been a battleground for naval skirmishes between the two Koreas because the North has never recognised the maritime border drawn unilaterally by the UN.
In July, it will be 60 years since North Korea and China signed an armistice with the US and the United Nations to bring an end to three years of fighting that cost millions of lives.
The designated Demilitarized Zone has evolved into the most heavily guarded border in the world. It was never intended to be a permanent one.
But six decades later, North and South remain divided, with Pyongyang feeling abandoned by the South in the quest for reunification and threatened by the Americans.
In that time, South Korea has blossomed from a poor, agrarian nation of peasants into the world's 15th-largest economy, while North Korea is struggling to find a way out of a Cold War chasm that has left it with a per-capita income on par with sub-Saharan Africa.
It's a tall order. Living standards in Pyongyang, the capital, are relatively high, with new shops and restaurants catering to a growing middle class.
But UN officials' reports detail harsh conditions elsewhere in North Korea: up to 200,000 people are estimated to be languishing in political prison camps, and two-thirds of the country's 24 million people face regular food shortages.
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