SEOUL, South Korea (AP) — North Korea has announced a detailed plan to launch a salvo of ballistic missiles toward the US Pacific territory of Guam, a major military hub and home to US bombers. If carried out, it would be the North’s most provocative missile launch to date.

The announcement Thursday warned that the North is finalising 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.

Japan and South Korea vowed a strong reaction if the North were to go through with the plan.

It said the plan, which involves the missiles hitting waters 30 to 40 kilometres (19 to 25 miles) from the island, could be sent to leader Kim Jong Un for approval within a week or so. It would be up to Kim whether the move is actually carried out.

It is unclear whether — or exactly why — North Korea would risk firing missiles so close to U.S. territory. Such a launch would almost compel the United States to attempt an intercept and possibly generate further escalation.

North Korea, no stranger to bluffing, 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 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, President Donald 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 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 ruling regime in Pyongyang.

Its reported plan is extremely specific, suggesting it is actually plotting a launch.

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 that could easily lead to severe consequences.

South Korea’s military responded by saying North Korea will face a “stern and strong” response from Washington and Seoul. Taking it a step further, Japan’s Defence Minister Itsunori Onodera told parliament a missile attack on the US territory would be a Japanese national emergency because it would threaten Japan’s existence as a nation.

If North Korea were to actually carry it out — even if it aimed at hitting the waters off Guam and not the island itself — that would clearly pose a potential threat to U.S. territory and put the United States in a much more complicated situation than it has been during previous missile launches.

Guam lies about 2,100 miles (3,400 kilometres) from the Korean Peninsula, and it’s extremely unlikely Kim’s government would risk annihilation with a pre-emptive attack on U.S. 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.

That 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 Hwasong-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 radius of more than 3,700 kilometres (2,300 miles). It can be fired from mobile launchers, making it hard to detect and destroy on the ground.

By launching a salvo of four, the North would be attempting to make it harder for the US to intercept all of the incoming missiles. Its stated flight path over Japan is also very aggressive — it has recently tried to avoid flying over neighbouring countries by shooting its missiles up at a very high angle to land in the ocean.

Washington, meanwhile, has been giving out mixed signals about its intentions.

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.

“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.”