North Korean leader Kim Jong Un's declaration that his country is no longer bound by its self-imposed nuclear and missile moratorium drastically raises the chances of longer-range weapons overflying Japan once again, experts say.

"Missile tests through Japanese airspace are only a matter of time now," said Van Jackson, a lecturer at Victoria University of Wellington and former Pentagon official.

The North last lobbed intermediate-range ballistic missiles (IRBMs) over Japan on two occasions in 2017, prompting the government to send out alarms on cellphones and interrupt television programs to urge residents to take cover.