The Japanese joint chiefs of staff said Wednesday's exercise with the US Air Force was conducted in "an increasingly severe security environment surrounding our country".

Tokyo said Wednesday that two Russian Tu-95 nuclear-capable bombers flew a day earlier from the Sea of Japan to rendezvous with two Chinese H-6 bombers in the East China Sea, then conducted a joint flight around the country.

Japan said that it scrambled fighter jets in response.

Prime Minister Sanae Takaichi angered Beijing last month by suggesting that Japan would intervene with military force in any Chinese attack on Taiwan.

Thursday's announcement by Japan's chiefs of staff said: "We confirmed the strong resolve of Japan and the United States not to allow any unilateral change of the status quo by force, as well as the readiness of the Self-Defense Forces and the US military."

In a separate statement it said that the "tactical exercises" over the Japan Sea involved two US B52 bombers, three Japanese F-35 fighter jets and three Japanese F-15s.

The joint exercise came as the United States criticized Beijing for the first time on Wednesday after Chinese military aircraft locked radar onto Japanese jets on Saturday.

The J-15 jets from China's Liaoning aircraft carrier twice locked radar on Japanese aircraft in international waters near Okinawa, according to Japan, which scrambled jets in response.

"China's actions are not conducive to regional peace and stability," a US State Department spokesperson told AFP on Wednesday.

"The U.S.-Japan Alliance is stronger and more united than ever. Our commitment to our ally Japan is unwavering, and we are in close contact on this and other issues."

Fighter jets use their radar for fire control to identify targets as well as for search and rescue operations.

Tokyo also summoned Beijing's ambassador following the radar incident, over which the two countries offer differing accounts of events.

Japan said it scrambled its F-15 jets because it was worried about possible "airspace violations".

Guo Jiakun, spokesman for the ministry of foreign affairs, accused Japan Wednesday of sending the jets "to intrude into the Chinese training area without authorisation, conduct close-range reconnaissance and harassment, create tense situations, and continue to maliciously hype up the situation".

Takaichi's comments about intervening in any Taiwan emergency enraged Beijing as China claims the self-ruled island as its own and has not ruled out seizing it by force.

Tokyo was forced to deny a Wall Street Journal report that said US President Donald Trump had advised Takaichi not to provoke China over Taiwan's sovereignty.

But Tokyo is apparently frustrated at the lack of public support from top officials in Washington and has urged the US to be more vocal, the Financial Times reported on Sunday.

- 'Regrettable' -

NATO chief Mark Rutte said on Wednesday that the radar incident and the joint Chinese-Russian patrols were "regrettable", Japan's Defence Minister Shinjiro Koizumi said on X.

The statement followed a 15-minute video conference between Rutte and Koizumi, the defence ministry said in a statement.

Rutte "affirmed that security in the Indo-Pacific and Euro-Atlantic regions is completely inseparable", Koizumi said.

South Korea said Tuesday that Russian and Chinese warplanes also entered its air defence zone, with Seoul also deploying fighter jets that same day.

Beijing confirmed later on Tuesday that it had organised drills with Russia's military according to "annual cooperation plans".

Moscow also described it as a routine exercise, saying it lasted eight hours and that some foreign fighter jets followed the Russian and Chinese aircraft.