Japan and NATO warned Tuesday against "growing" military proximity between Russia and China, pledging to beef up their security partnership amid Moscow's war in Ukraine and Beijing's military buildup in the Indo-Pacific region.

Prime Minister Fumio Kishida and NATO Secretary General Jens Stoltenberg reached the agreement at their meeting in Tokyo, as Japan and other regional democracies have been reinforcing security relations with the trans-Atlantic alliance, led by the United States.

"We highlight with concern Russia's growing military cooperation with China, including through joint operations and drills in the vicinity of Japan," the leaders said in a joint statement released after their talks.

Japanese Prime Minister Fumio Kishida (R) and NATO Secretary General Jens Stoltenberg hold a joint press conference at the Japanese premier's office in Tokyo on Jan. 31, 2023. (Kyodo)

"We recognize that the security of the Euro-Atlantic and of the Indo-Pacific is closely connected and stress the necessity of further strengthening cooperation between Japan and NATO in order to respond to the changing strategic environment," they added.

Japan said Russian and Chinese bombers jointly flew above waters near the country in May and November last year, with Moscow and Beijing apparently testing the response capabilities of the Tokyo-Washington alliance.

Kishida and Stoltenberg strongly opposed "any unilateral attempts to change the status quo by force or coercion," given the recognition that the free and open international order based on the rule of law is "at stake."

"I welcome NATO's deepening interest and involvement in the Indo-Pacific region," Kishida said at a joint press event with Stoltenberg after their talks.

Kishida added Japan will set up a mission for the Brussels-based North Atlantic Treaty Organization to promote closer communication.

NATO Secretary General Jens Stoltenberg (L, front), and Japan's Prime Minister Fumio Kishida (R, front) hold talks at the premier's office in Tokyo on Jan. 31, 2023. (Kyodo) ==Kyodo

The two leaders confirmed that Japan and NATO will continue supporting Ukraine while condemning Russia's invasion.

They emphasized that Moscow's "irresponsible" rhetoric is unacceptable and any nuclear weapon use would lead to "severe consequences."

With fears lingering that Russian President Vladimir Putin might use a nuclear device in the course of his war launched in February last year, worries have also been mounting about China's possible military action against Taiwan.

Communist-led China considers the self-ruled democratic island as a renegade province to be unified with the mainland, by force if necessary. The two sides of the Taiwan Strait have been governed separately since 1949 due to a civil war.

Kishida and Stoltenberg said peace and stability across the strait are important "as an indispensable element in security and prosperity" in the international community while urging China to improve transparency regarding its "rapid" military expansion.

Stoltenberg said China is "substantially building up its military forces, including nuclear weapons, bullying its neighbors and threatening Taiwan" and "spreading disinformation about NATO and the war in Ukraine."

China has lambasted the United States and its security allies for striving to establish an "anti-China alliance in Asia" and eventually form an "Asian NATO."

Kishida and Stoltenberg also agreed it is crucial to boost cooperation in cyberspace, outer space, disinformation and other fields to bolster their capabilities "to address emerging challenges in new domains."

NATO views Japan as a partner nation, along with South Korea, Australia and New Zealand. The countries' leaders were invited for the first time to a NATO summit in Madrid in June 2022.

Before the ongoing war, Russia had been complaining about NATO's military activities near its border and stepping up its demands for security guarantees, such as precluding NATO's expansion eastward to Ukraine.

After visiting South Korea and meeting with President Yoon Suk Yeol, Stoltenberg, a former Norwegian prime minister, has stayed in Japan since Monday. He is scheduled to leave Asia Wednesday.