A Chinese H-6 bomber flies over East China Sea.
Chinese and Russian fighter jets carried out joint flights near Japan on Tuesday as leaders of the Quad bloc met in Tokyo, the Japanese defence minister said.
Nobuo Kishi said the government had expressed "grave concerns" to Russia and China over the flights, which took place while leaders from the United States, India, Australia and Japan held talks on regional security.
The planes did not breach territorial airspace, the defence ministry told AFP, and it is the fourth time since November that long-distance joint flights by Russia and China have been spotted near Japan.
"Two Chinese bombers joined two Russian bombers in the Sea of Japan and made a joint flight to the East China Sea," Kishi told reporters.
"After that, a total of four aircraft, two presumed (new) Chinese bombers -- which replaced the two Chinese bombers -- and two Russian bombers, conducted a joint flight from the East China Sea to the Pacific Ocean."
He said a Russian intelligence-gathering aircraft also flew off northern Hokkaido to the Noto Peninsula in central Japan on Tuesday, calling the moves especially "provocative" given the summit in Tokyo.
The Quad leaders on Tuesday warned against attempts to "change the status quo by force", although they avoided direct references to Russia or China in a joint statement.
Their statement referred to the war in Ukraine, however, and listed a range of activities that Beijing has regularly been accused of in the region.
Kishi said Japan had "communicated through diplomatic routes our grave concerns from the perspective of our country's and the region's security".
"As the international community responds to Russia's aggression against Ukraine, the fact that China took such action in collaboration with Russia, which is the aggressor, is cause for concern. It cannot be overlooked."
Japan, which has volatile ties and border disputes with its neighbours China, Russia and South Korea, routinely sends jets to defend its air borders.
The country scrambled military jets 1,004 times in the past year to March, the second highest on record, according to the defence ministry.
Most were to counter approaching Chinese aircraft, although many of the rest involved Russian aircraft.
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