The military budget has been rising steadily for about a decade, and the defence ministry's latest request warns that "the security environment surrounding our country is increasingly severe."

The ministry is asking for 5.48 trillion yen ($50 billion) in the fiscal year from April 2022, up from the 5.3 trillion yen that was eventually approved for the ongoing year to March.

The figure is slightly down though from the amount the ministry initially requested last year, a record 5.49 trillion yen.

The budget request envisions Japan building five military ships and a submarine, and purchasing 12 F-35 fighter jets.

The ministry also said it aims to boost the nation's defensive capabilities in space and cybersecurity.

Tokyo's military readiness is mainly concerned with growing threats from Beijing and Pyongyang, said Hideshi Takesada, a defence expert and visiting professor at Takushoku University.

"The military strength of the People's Liberation Army of China is growing faster than we thought ... while threats from North Korea's missile and nuclear development have never really diminished," Takesada told AFP.

China's military spending has risen in tandem with its growing economy and has seen an increase for 26 consecutive years, reaching an estimated $252 billion in 2020, researchers have said.

Meanwhile, the International Atomic Energy Agency says North Korea appears to have restarted its plutonium-producing reprocessing reactor in a "deeply troubling" development, a possible sign Pyongyang is expanding its banned weapons programme.

Japan's growing defence budget is also the result of pressure from the United States to play a bigger role in regional security as US-China tensions rise, Takesada said.

"As long as Japan's current conservative government remains in power, the nation's defence budget is expected to remain high," he added.

In an annual defence paper released last month, Japan said US-China tensions over Taiwan are an increasingly urgent issue that threatens regional stability.

Beijing considers Taiwan part of its territory and has ramped up diplomatic, military and economic pressure on the self-governed island in recent years.

The United States has reacted strongly to Beijing's pressure campaign, putting US ally Japan in a tough position between two world powers that are both key trade partners.

But Japan has been increasingly vocal about China's maritime expansion and military build-up, publicly protesting about the presence of Chinese vessels around disputed islets known as the Senkaku by Tokyo and the Diaoyu by Beijing.

This month, Taiwan and Japan's ruling parties organised security talks for the first time, in a moved that was condemned by Beijing.