Speaking in Tokyo, Jens Stoltenberg said Japan's renewed focus on security made the nation "even more" of a partner "for peace".

"I am glad that Japan is planning (a military budget) to reach the NATO benchmark of two percent of GDP devoted to defence," he said in an address at Keio University.

For decades, Japan has capped military spending at around one percent of GDP, but late last year Prime Minister Fumio Kishida's government approved a new security strategy, including plans to increase defence spending to two percent of GDP by fiscal year 2027.

"This demonstrates that Japan takes international security seriously," Stoltenberg said.

Rising threats from China and North Korea, as well as Russia's war on Ukraine, have fuelled public support for greater military spending in Japan.

Stoltenberg said Russian President Vladimir Putin had "totally underestimated Ukraine" and the strong unity among NATO members in supporting the country.

While stressing NATO did not see China as an adversary, he said Beijing's military build-up and ambitions were an issue to address.

"Today, the global order that has served us so well for so many decades is under threat. Moscow and Beijing are at the forefronts of... authoritarian pushback," Stoltenberg said.

"In a more dangerous world, Japan can count on NATO to stand with you," he added.

Stoltenberg met Kishida on Tuesday and said afterwards they had agreed to "remain united and firm" in the face of security threats from China, North Korea and the Ukraine war.

Asked about Stoltenberg's comments, Chinese foreign ministry spokeswoman Mao Ning said Wednesday that "NATO should seriously consider the role it has played in maintaining European security".

"On one hand, NATO claims that its position as a regional and defensive alliance has not changed," she said at a regular briefing.

"On the other hand, it continues to push beyond its traditional defence zone and territory, continuously strengthens military security ties with Asia-Pacific countries, and exaggerates the China threat."