The policy section chief of the Institute for Japan Studies under the foreign ministry described the Japanese defence white paper, approved this week, as "a war scenario for realizing its ambition for reinvasion from A to Z," the Korean Central News Agency said.

The white paper was approved by the cabinet of Prime Minister Shigeru Ishiba on Tuesday.

It argued, among other things, that China's intensifying military activities could seriously impact Japanese security, citing the first confirmed incursion by a Chinese military aircraft into its airspace.

It also said North Korea's activities pose a "more grave and imminent threat to Japan's national security than ever before".

Japan is in a multi-year process of increasing its defence spending, as many countries are doing under pressure from the United States, as Donald Trump governs for a second time with a focus on more-burden sharing on defence.

Japan is bolstering its military ties with Washington -- and other regional US allies -- to make US and Japanese forces nimbler in response to threats such as a potential Chinese invasion of Taiwan.

The Japan studies department of the North Korean foreign ministry said Japan was "escalating the regional situation in a gradual way (to) justify its reckless moves to turn itself into a military giant," KCNA reported.

Japan's military activities show that North Korea's efforts to build up its nuclear arsenal "serve as an indispensable contribution to strongly suppressing the provocations of the US and its allies," the agency reported.