Seoul – North Korea on Thursday announced a detailed plan to launch a volley of ballistic missiles toward the US Pacific territory of Guam, a major military hub and home to US bombers, and dismissed President Donald Trump's threats of "fire and fury" if it doesn't back down.The announcement, made in the name of a general who heads North Korea's rocket command, warned the North is preparing a plan to fire four of its Hwasong-12 missiles over Japan and into waters around the tiny island, which hosts 7 000 US military personnel on two main bases and has a population of 160 000.It said the plan could be finalised within a week or so and would then go to leader Kim Jong-Un for approval. It would be up to Kim whether the move is actually carried out. It said the missiles would hit waters 30km to 40km away from the island.It is unclear whether North Korea would risk firing missiles so close to US territory, which could provoke counter-measures and further escalation.North Korea frequently uses extremely bellicose rhetoric with warnings of military action to keep its adversaries on their heels. It generally couches its threats with language stating it will not attack the United States unless it has been attacked first or has determined an attack is imminent.But the statement raised worries amid a barrage of threats from both sides.Following reports that US intelligence suggests the North might be able to pair a nuclear warhead with a missile capable of reaching targets on the United States mainland, Trump warned North Korea that "it faces retaliation with fire and fury unlike any the world has seen before".Pyongyang, meanwhile, has been louder in its complaints against a new and tough round of sanctions imposed on it by the United Nations, with strong US backing, and Washington's use of Guam as a staging ground for its stealth bombers, which could be used to attack North Korea and are a particularly sore point with the rulers in Pyongyang.Military capabilitiesEven so, its reported plan to launch missiles toward Guam is extremely unusual.The report said the Hwasong-12 rockets would fly over Shimane, Hiroshima and Koichi prefectures in Japan and travel "1 065 seconds before hitting the waters 30 to 40 kilometres away from Guam".It said the Korean People's Army Strategic Force will finalise the plan by mid-August, present it to Kim Jong-Un and "wait for his order"."We keep closely watching the speech and behaviour of the US," it said.Such a move would not merely be a test launch, but a demonstration of military capabilities in a manner that could easily lead to severe consequences.If North Korea were to actually carry out such a launch – even if it aimed at hitting the waters off the island and not the island itself – that would clearly pose a potential threat to US territory and put the United States in a much more complicated situation than it has been during previous missile launches.Guam lies about 3 400km from the Korean Peninsula, and it's extremely unlikely Kim's government would risk annihilation with a pre-emptive attack on US citizens. It's also unclear how reliable North Korea's missiles would be against such a distant target, but no one was writing off the danger completely.Washington has been testing its missile defences in response to the North's stepped-up development and the current escalation of tensions could lead to pressure for the US military to try to shoot down the North's missiles in mid-flight if they are heading toward Guam.Mixed signalsThat would likely open up a set of very major problems, including the possibility of both a very high-profile failure or a miscalculation of Washington's intentions and a more deadly pre-emptive strike by the North – which has missiles able to hit Tokyo and conventional weapons that could devastate South Korea's capital of Seoul.The Hweasong-12, which was revealed for the first time at a military parade in April, is an intermediate-range ballistic missile that is believed to have a range of more than 3 700km. It can be fired from mobile launchers, making it hard to detect and destroy on the ground.By launching a volley of four, the North would be attempting to make it harder for the US to intercept all of the incoming missiles.Washington, meanwhile, has been giving out mixed signals of what its intentions might be.While Trump was threatening annihilation and boasting from the New Jersey golf resort where he's vacationing that he has made the US nuclear arsenal "far stronger and more powerful than ever before", Secretary of State Rex Tillerson sought to calm the sense of crisis.Speaking earlier on Wednesday on his way home from Asia, he insisted the US isn't signalling a move toward military action."Americans should sleep well at night," Tillerson told reporters. "Nothing that I have seen and nothing that I know of would indicate that the situation has dramatically changed in the last 24 hours."But then Defence Secretary Jim Mattis ratcheted the rhetoric back up, calling on Pyongyang to "cease any consideration of actions that would lead to the end of its regime and the destruction of its people".As seldom as it is for a president to speak of using nuclear missiles, the reference to the "destruction" of a foreign people is equally rare.North Korea immediately called Trump's rhetoric a "load of nonsense" that was aggravating a grave situation.