TOKYO - North Korea has already miniaturised nuclear weapons that can be fitted on ballistic missile warheads, Japan acknowledged for the first time in its annual defence report issued on Friday (Sept 27).

In doing so, Tokyo stressed that the North remains a grave and imminent threat in the midst of stop-start denuclearisation talks between Pyongyang and Washington.

The release of the White Paper coincided with a statement by North Korea's Foreign Ministry adviser Kim Kye Gwan that the lack of progress in carrying out past summit agreements has cast doubt on the prospects for a future summit between US President Donald Trump and North Korean leader Kim Jong Un.

Japan had, in its past editions of the White Paper, only broached the possibility of Pyongyang miniaturising its warheads.

But the firmer language this time took note of the "technical maturity obtained" through six past nuclear tests, and a series of 10 short-range ballistic missile tests this year.

"North Korea possesses and deploys several hundred ballistic missiles capable of reaching every part of Japan and continues to possess capabilities for conducting surprise attacks against Japan," it said.

Further, Japan assessed that North Korea possesses large-scale cyber units and special operations forces, which are responsible for the theft of military secrets, the development of capabilities to attack critical infrastructure of foreign countries, and guerrilla warfare.

These units are among the 1.28 million active personnel that North Korea has in its military, the report said. It also noted that the North has about 3,500 tanks, 780 naval vessels and 550 combat aircraft, though most are outdated.

Japan sees North Korea as working to diversify the forms of missile launches, build its capability for surprise strikes, and enhance its ability to conduct "saturation attacks", or the swarming of defences with simultaneous missile launches.