North Korea nuclear war threat: US troops 'poised to respond' as madman Kim Jong Un says attack is 'authorised' N Korea has moved what appears to be a Musudan missile to its east coat U.S deploys missile defence system to Pacific island of Guam 'The moment of explosion is approaching fast,' warned North Korea U.S. Defense Secretary Chuck Hagel: 'Real and clear danger' from North North Korea's Twitter account has been hacked David Cameron: North Korea is an 'unpredictable and aggressive regime' North Korea today warned "the moment of explosion is near' as it declared that troops have been cleared to 'mercilessly' attack the U.S. using 'smaller, lighter and diversified' nuclear weapons. The rogue state has moved a missile with a range of 3,000km (1,800m) to its east coast - within range of Japan - and claimed it would be "merciless" against its enemies. Kim Jong Un's dramatic deployment came after the U.S. announced it was sending ballistic missile defences to Guam - the tiny Pacific Island that is among a list of possible targets for attack including Hawaii and the U.S. mainland. The Pentagon said the anti-missile shield would be ready within weeks to address the 'real and clear danger' that the communist state presents to the U.S. and its allies. The strident warning from Pyongyang is the most serious so far from North Korea. A map showing the movement in the region. North Korea warned today that its military has been cleared to attack the U.S. using 'smaller, lighter and diversified' nuclear weapons as the Pentagon announced it is deploying a missile to the Pacific island of Guam Despite the intense rhetoric, analysts do not expect a nuclear attack by North Korea, which knows the move could trigger a destructive, suicidal war. Experts believe Pyongyang does not yet have the ability to launch nuclear-tipped missiles, but its other nuclear capabilities aren't fully known. But the statement from North Korea are the most serious so far and triggered worldwide concern. The military statement read they are authorised to wage 'cutting-edge smaller, lighter and diversified' nuclear strikes to protect against the United States. The moment of explosion is approaching fast. No one can say a war will break out in Korea or not and whether it will break out today or tomorrow,' read the statement of an unnamed military spokesman. The statement, which was carried by the state-run Korean Central News Agency (KCNA), stated that 'the merciless operation of its revolutionary armed forces in this regard has been finally examined and ratified.' Kitted out: A soldier of a South Korean artillery unit looks at his fellow soldiers and military vehicles moving to conduct military training near the demilitarized zone separating the two Koreas in Paju Ready: A South Korean artillery unit drive a self-propelled artillery vehicle for training Drills: South Korean soldiers ride the tanks through the streets on their way to take part in military drills Watchful eye: A North Korean soldier looks through binoculars at the truce village of Panmunjom in the demilitarized zone dividing the two Koreas Is North Korea's missile system a threat? Today South Korea's defence minister said that North Korea has moved a missile with 'considerable range' to its east coast. Officials said there were no signs the North Koreans planned to test fire their new longer-range KN-08 road-mobile intercontinental ballistic missile. The range he described could refer to a mobile North Korean missile known as the Musudan. What is the Musudan? The Musudan, also known as the Nodong-B or the Taepodong-X, is an intermediate-range ballistic missile. It has a range of 3,000 kilometres (1,800 miles) - that would make Japan and South Korea potential targets, but little is known about the missile's accuracy. What is the KN-08? Adm. James Winnefeld, vice chairman of the Joint Chiefs of Staff, noted this missile could probably hit the U.S - it is thought he means Alaska. Defence Secretary Chuck Hagel cited the KN-08 as a reason for the administration's decision to deploy 14 additional missile interceptors in Alaska. The North Korean military claims it has a range of 10,000 kilometres, but military experts believe that the 18-metre-long KN-08 has a range of 6,000 kilometres. It is a concern as it is road-mobile and easy to hide. Is the U.S. strong enough? North Korea isn't thought to have nuclear-armed missiles that can hit the United States and is extremely unlikely to launch a direct attack on Seoul or its U.S. ally, knowing that military retaliation would threaten the leadership's survival. This week the Pentagon deployed two Aegis-equipped missile defense ships to operate off the Korean peninsula. It is readying long-range ground-based interceptors in Alaska and California. It is also deploying one of its newest ground-based missile defences to the Pacific. What about the nuclear power station? North Korea's vow to restart its mothballed nuclear facilities raises fears about assembly lines churning out fuel for a fearsome arsenal of nuclear missiles. But it may actually be a sign that Pyongyang needs a lot more bomb fuel to back up its nuclear threats. Despite the bluster, it could be years before North Korea completes the laborious process of creating more weaponized fuel. Its announcement, experts say, is also likely an effort to boost fears meant to keep its leadership safe while trying to extract concessions from the U.S. and its allies. North Korea is reacting against joint U.S. and South Korean military exercises taking place in South Korea and has expressed anger over tightened sanctions for its February nuclear test. Today South Korea's defence minister said that North Korea has moved a missile with 'considerable range' to its east coast, but he added that there are no signs that Pyongyang is preparing for a full-scale conflict. South Korean Defense Minister Kim Kwan-jin dismissed reports in Japanese media that the missile could be a KN-08, which is believed to be a long-range missile that if operable could hit the United States. Kim told lawmakers at a parliamentary committee meeting that the missile has 'considerable range' but not enough to hit the U.S. mainland. The range he described could refer to a mobile North Korean missile known as the Musudan, which has a range of 3,000 kilometers (1,800 miles). That would make Japan and South Korea potential targets, but little is known about the missile's accuracy. The defence minister said he did not know the reasons behind the missile movement, saying it 'could be for testing or drills.' Experts say North Korea has not demonstrated that it has missiles capable of long range or accuracy. Some suspect that long-range missiles unveiled by Pyongyang at a parade last year were actually mockups. Kim Kwan-jin said that if North Korea were preparing for a full-scale conflict, there would be signs including the mobilization of a number of units, including supply and rear troops, but South Korean military officials have found no such preparations. ' (North Korea's recent threats) are rhetorical threats. I believe the odds of a full-scale provocation are small,' he said. But he added that there is still the possibility of North Korea mounting a localized, small-scale provocation against South Korea. He cited the 2010 shelling of a South Korean island, an attack that killed four people, as a possible example of such a provocation. The movement of the missile is thought to be in response to the U.S. deploying an anti-missile shield to Pacific to strengthen regional protection against a possible attack. The Terminal High-Altitude Area Defense system is expected to arrive at the U.S. territory in the Pacific within two weeks. The $800m (£529m) battery was not due for deployment until 2015, but a Pentagon statement said the deployment was 'a precautionary move to strengthen our regional defence posture against the North Korean regional ballistic missile threat.' The deployment of the battery is a firm demonstration that Washington regards the confrontation with North Korea as more worrying than similar crises in the past and that they are preparing for a long standoff. U.S. Defence Secretary Chuck Hagel said Washington is doing all it can to defuse the situation, echoing comments a day earlier by Secretary of State John Kerry. 'Some of the actions they've taken over the last few weeks present a real and clear danger and threat to the interests, certainly of our allies, starting with South Korea and Japan, and also the threats that the North Koreans have leveled directly at the United States regarding our base in Guam, threatened Hawaii, threatened the West Coast of the United States,' Hagel said yesterday. 'They have a nuclear capacity now,' he told the National Defense University in Washington. They have a missile delivery capacity now. And so, as they have ratcheted up their bellicose, dangerous rhetoric, and some of the actions they have taken over the last few weeks, present a real and clear danger. Yesterday a Chinese diplomat met ambassadors from the United States and both Koreas to express 'serious concern' over the situation on the Korean peninsula, China's Foreign Ministry said. 'Yesterday afternoon, Deputy Foreign Minister Zhang Yesui met the ambassadors of North and South Korea and the United States and expressed serious concern about the present situation on the peninsula,' Chinese Foreign Ministry spokesman Hong Lei told reporters at a daily briefing. 'In the present situation, China believes all sides must remain calm and exercise restraint and not take actions which are mutually provocative and must certainly not take actions which will worsen the situation,' he said. VIDEO: U.S. army demonstrates decontamination exercise in South Korea Kim Jong Un, was unveiled in September 2010 as his father's choice as successor, and is the third-generation Kim to rule the nation of 24 million. In April 2012 he formally took over ruling party leadership, becoming First Secretary of the Workers Party. He is believed to be in his late 20s but even his exact age is unknown outside the North. The nation is notoriously secretive - no official announcement was made that Kim Jong Un even had a wife until state media eventually revealed it. According to South Korean media, citing intelligence reports, the couple were married in 2009. It was claimed last month that North Korea has a new communist heir after his wife secretly gave birth late last year. The newborn, whose gender is not known, could become the fourth-generation to lead the communist dictatorship, but Pyongyang watchers believe the baby is most likely a girl. 'If it was a boy, [the North Koreans] would have made an announcement,' Michael Madden, editor of the online newsletter North Korea Leadership Watch, told the Washington Free Beacon. Analysts say the threats are probably efforts to provoke softer policies from South Korea, to win diplomatic talks with Washington and to solidify the image of young North Korean leader Kim Jong Un. Hackers have apparently disrupted North Korea's government-run Twitter account. The disruption comes at a time of rising tensions on the Korean Peninsula. The North's Uriminzokkiri's Twitter account on Thursday displayed four tweets saying 'hacked.' A fifth tweet said 'Tango Down' and was followed by a link to Uriminzokkiri's Flickr page. Both sites were running today, but carrying content that differed sharply from content typically posted by the regime in Pyongyang, leading viewers to assume the accounts had been hacked. North Korea opened its Uriminzokkiri Twitter account in 2010 to use the social media to praise its system and leaders. At times Pyongyang has gone beyond rhetoric. For a second day running this morning North Korean border authorities denied entry to South Koreans who manage jointly run factories in the North Korean city of Kaesong. A North Korean government-run committee threatened to pull out North Korean workers from Kaesong as well. On Tuesday, Pyongyang announced it would restart a plutonium reactor it had shut down in 2007. A U.S. research institute said yesterday that satellite imagery shows that construction needed for the restart has already begun. North Korea's military statement today said its troops had been authorized to counter U.S. 'aggression' with 'powerful practical military counteractions,' including nuclear weapons. South Korea's Defense Ministry said its military is ready to deal with any provocation by North Korea. 'I can say we have no problem in crisis management,' deputy ministry spokesman Wee Yong-sub told reporters. Worst-case scenario: North Korea's best hope to score a strike on a U.S. territory would be to fire the Taepodong-2 rocket, which could possibly make it to Alaska Nascent technology: A map from the Federation of American Scientists and the Center for Non-proliferation Studies shows that North Korea's missiles cannot even reach India about 3,100 miles away Deployed: USS John McCain a guided-missile destroyer used for ballistic missile defence, has been put into position to operate off the Korean peninsula This spring's annual U.S.-South Korea drills have incorporated fighter jets and nuclear-capable stealth bombers, though the allies insist they are routine exercises. Pyongyang calls them rehearsals for a northward invasion. The foes fought on opposite sides of the Korean War, which ended in a truce in 1953. The divided Korean Peninsula remains in a technical state of war six decades later, and Washington keeps 28,500 troops in South Korea to protect its ally. North Korea's nuclear strike capabilities remain unclear. Pyongyang is believed to be working toward building an atomic bomb small enough to mount on a long-range missile. Long-range rocket launches designed to send satellites into space in 2009 and 2012 were widely considered covert tests of missile technology, and North Korea has conducted three underground nuclear tests, most recently in February. 'I don't believe North Korea has the capacity to attack the United States with nuclear weapons mounted on missiles, and won't for many years. Its ability to target and strike South Korea is also very limited,' nuclear scientist Siegfried Hecker, a senior fellow at the Center for International Security and Cooperation at Stanford University, said this week. 'And even if Pyongyang had the technical means, why would the regime want to launch a nuclear attack when it fully knows that any use of nuclear weapons would result in a devastating military response and would spell the end of the regime?' he said in answers posted to CISAC's website. In Seoul, a senior government official said Tuesday it wasn't clear how advanced North Korea's nuclear weapons capabilities are. But he also noted fallout from any nuclear strike on Seoul or beyond would threaten Pyongyang as well, making a strike unlikely. He spoke on condition of anonymity because he wasn't authorized to speak publicly to the media. David Cameron issued a stark warning against any move to abandon Britain's Trident deterrent in the face of the growing nuclear threat from North Korea and Iran. The Prime Minister said it would be 'foolish' to leave the country defenceless at a time when the 'highly unpredictable and aggressive' regime in North Korea was developing ballistic missiles which could eventually threaten Europe. Writing in The Daily Telegraph, Mr Cameron said such 'evolving threats' underlined the need for the UK to maintain the ultimate deterrent. We need our nuclear deterrent as much today as we did when a previous British government embarked on it over six decades ago. But the nuclear threat has not gone away,' he said. 'The highly unpredictable and aggressive regime in North Korea recently conducted its third nuclear test and could already have enough fissile material to produce more than a dozen nuclear weapons,' he said. North Korea maintains that it needs to build nuclear weapons to defend itself against the United States. On Monday, North Korean leader Kim Jong Un led a high-level meeting of party officials who declared building the economy and 'nuclear armed forces' as the nation's two top priorities. Hecker has estimated that North Korea has enough plutonium to make several crude nuclear bombs. Its announcement Tuesday that it would restart a plutonium reactor indicated that it intends to produce more nuclear weapons material. The U.S.-Korea Institute at Johns Hopkins School of Advanced International Studies has analyzed recent commercial satellite imagery of the Nyongbyon nuclear facility, where the reactor was shut down in 2007 under the terms of a disarmament agreement. A cooling tower for the reactor was destroyed in 2008. The analysis published yesterday on the institute's website, 38 North, says that rebuilding the tower would take six months, but a March 27 photo shows building work may have started for an alternative cooling system that could take just weeks. Experts estimate it could take three months to a year to restart the plant. Threat: Kim Jong Un, pictured centre at a meeting of North Korea's parliament this week, has vowed to re-open the country's nuclear facilities