Japanese Prime Minister Shinzo Abe says the government is on high alert to deal with the possible threat faced by the prefectures over which North Korean missiles en route to Guam could fly.
"We will do our utmost to ensure no harm will befall our citizens," he reassured the governors of Shimane, Hiroshima, Kochi and Ehime on Monday, two days after Patriot Advanced Capability-3 (PAC-3) missile interceptor systems were deployed in their prefectures.
The governors had sought Tokyo's "continued effort to strengthen its vigilance and defence system" in a petition.
The four prefectures were not previously covered by the PAC-3 system. Japan owns 34 PAC-3 units and usually deploys them in densely populated areas such as Tokyo and at major military bases.
Defence Minister Itsunori Onodera said last week that it is within the pacifist nation's constitutional rights to shoot down any missiles, as well as their debris, flying over Japan. He said Japan can legally intercept any missile that flies over its territory that could threaten its ally, the United States, under its principle of collective self-defence.
But current affairs website The Diplomat pointed out in a commentary last week that the US and Japan have traditionally not intercepted any North Korean missiles to mine useful data and avoid "unnecessary escalation". The risk, the article said, was that if any US attempts at interception were to fail, it will raise doubts over its missile defences in the eyes of its allies.
North Korean state media last week reported plans to launch four Hwasong-12 intermediate-range ballistic missiles that will "cross the sky above Shimane, Hiroshima and Kochi prefectures". While Ehime was not cited, it neighbours the latter two prefectures.
If its plan is carried out, it will be the first time that North Korea has lobbed a missile over Japan's mainland since April 2009, when one rocket flew over north-eastern Akita and Iwate prefectures before falling into the Pacific Ocean. The Self-Defence Force made no bid to shoot down that rocket because its debris did not threaten Japan.
The land-based PAC-3 is the second layer of Japan's missile defence system, and can destroy targets at altitudes of between 10km and 20km.
They add to the first layer of Standard Missile-3 (SM-3) units, which are mounted on Aegis warships patrolling waters around Japan round the clock and can intercept ballistic missiles outside the earth's atmosphere up to an altitude of 500km.
Any missile fired by North Korea towards Guam will still be on its ascent as it flies over Japan, defence experts said, which means the SM-3 might stand a better shot of intercepting the rocket.
But the PAC-3 can shoot down stray missiles that could land in Japan perhaps due to a mid-flight failure, as well as rocket debris.
Still, any "swarm tactic" of multiple missiles at the same time could overwhelm Japan's defence, noted Kobe University security expert Tosh Minohara. "There are still too many gaping holes in Japan's air defence, and there are not enough PAC-3 units," he said.
Mr Onodera and Foreign Minister Taro Kono will meet their US counterparts James Mattis and Rex Tillerson in Washington tomorrow, with missile defence capabilities likely on the agenda.