Japan and South Korea will come to America's aid if North Korea attacks Guam

Japanese defence minister said it would be legal for Japan to intercept a missile aimed at Guam.

TV images of President Donald Trump and North Korean leade Kim Jong-un on a South Korean news programme on August 10, 2017. Ahn Young-joon / AP

South Korea and Japan warned North Korea it would face a strong response if it carried through with a threat to launch a missile towards the US territory of Guam.

North Korea on Thursday  announced a detailed plan to launch a salvo of ballistic missiles toward the Pacific island , which - if carried out -  carried out would be its most provocative missile launch to date.

Japanese defence minister Itsunori Onodera said it would be legal for Japan to intercept a missile aimed at Guam. A spokesman for south Korea's joint chiefs of staff said North Korea's threats to strike around Guam pose a serious challenge.

“We give a strict warning,” the spokesman said. “If North Korea commits provocations despite our stern warning, it will face a strong response from South Korea’s military and the US-South Korea alliance.”

Later on Thursday, South Korea’s National Security Council took a softer tone, saying the door for dialogue remained open and it would take all possible steps to ease tensions.

North Korea, responding to US President Donald Trump's "fire and fury" warning, outlined a detailed plan to fire four Hwasong-12 intermediate-range ballistic missiles at Guam by mid-August. They would fly over southwestern Japan and land as close as 30 kilometres from Guam, the Korean Central News Agency (KCNA) reported on Thursday.

“Sound dialogue is not possible with such a guy bereft of reason and only absolute force can work on him,” KCNA reported, citing a statement by a general of the Korean People's Army in response to  Mr Trump . “The military action the KPA is about to take will be an effective remedy for restraining the frantic moves of the US in the southern part of the Korean peninsula and its vicinity.”

More than 3,000 kilometres southeast of North Korea, Guam is an important strategic hub for the American military, hosting  air and naval bases, as well as missile-defence launchers on the island.

US secretary of state Rex Tillerson sought to calm tensions after Trump’s remarks, saying “Americans should sleep well at night.” Later, defence secretary James Mattis warned North Korea  it would lose any conflict it initiates.

North Korea first fired a missile over Japan in 1998, prompting the Japanese government to initiate its current ballistic missile-defense system with the US. While a second attempt failed in 2005, North Korea again succeeded in launching one in 2009 that flew over northern Japan and continued for another 3,000 kilometers before landing in the Pacific.

More recently, North Korean missiles have landed in the Sea of Japan, with some falling in Japan’s exclusive economic zone that stretches as much as 200 nautical miles from its shores.

Japan passed legislation two years ago allowing it to come to the aid of another country in certain circumstances, under a reinterpretation of its pacifist constitution. The defence minister, told parliament on Thursday that an attack on Guam would fall under that legislation because of its importance to Japan's own defence.

But it is  unclear if Japan could shoot down a missile heading to Guam.

"My initial impression would be not really," said Lance Gatling, president of Nexial Research, an aeropsace consultancy in Tokyo. "A ballistic missile going to Guam is flying very high, going very fast and accelerating by the time it's over Japan. Without prior knowledge that it's coming, you are reduced to chasing it, which is a great disadvantage."

A willingness to shoot down the missile was a politically important move in the relationship between the two allies, Mr Gatling said, adding that Japan might be better placed to provide data to help the US track the missile.

Japan has a two-layered ballistic missile defense system, consisting of ship-based SM-3 interceptors, which are intended to shoot down missiles outside the Earth’s atmosphere, and land-based Patriot interceptors, which are designed to intercept them in the final stage of their trajectory. It is also looking into the possibility of adding a third missile defense element.

Japan’s chief cabinet secretary Yoshihide Suga also underscored the importance of the alliance, which he said would be further strengthened through a meeting between Mr Tillerson, Mr Mattis and their Japanese counterparts in Washington next week.

“North Korea’s provocative actions, including on this occasion, are a clear threat to the region and the international community and can absolutely not be tolerated,”he said.