Japan says in its annual defense report that China may move up its plan to build a "world-class military" by the mid-21st century as Beijing is building up and modernizing its military, a draft of the paper showed Tuesday.

In the draft 2023 edition of the defense white paper, Japan expresses grave concerns regarding increasing Russian and Chinese military activities in waters around the country, saying they seem "to clearly intend a show of force to our country."

Photo taken on June 15, 2020, shows the headquarters of Japan's Defense Ministry in Tokyo. (Kyodo)

The draft points out that China and Russia have conducted a total of five joint bomber flights near Japan since July 2019.

Of the five cases, it said a flight in May 2022, when Tokyo was hosting a summit of the Quad group, also involving Australia, India and the United States, was a "show of force."

The paper is set to be formally reported at a Cabinet meeting and released in July.

The draft describes China as its "greatest strategic challenge," repeating a phrase in the National Security Strategy, updated in December last year.

The paper estimates that China could possess 1,500 nuclear warheads by 2035 and says the country is focused on seeking dominance in new frontiers, including outer space, cyberspace and electromagnetic warfare, according to the draft.

Chinese leader Xi Jinping vowed to build a "world-class military" by the mid-21st century at the National Congress of the Chinese Communist Party in 2017.

The draft paper says China may move that deadline forward, noting that Xi called for a rapid elevation of the People's Liberation Army to world-class standards in his opening speech at the party's congress in October last year.