Japan says it had to scramble jets after Russian and Chinese warplanes neared its airspace while the country was hosting leaders from Australia, the United States and India.
At least four Russian planes also entered South Korea’s air defence zone on Tuesday.
The aircraft – Russian Tu-95 strategic bombers and Chinese Xian H-6 jets – were capable of carrying nuclear weapons, according to the Financial Times.
Tokyo conveyed “grave concerns” through diplomatic channels in response to what has been viewed as a likely provocation by Beijing and Moscow, according to Japanese Defence Minister Nobuo Kishi.
“We believe the fact that this action was taken during the Quad summit makes it more provocative than in the past,” he said.
Mr Kishi said the “unacceptable” flyby was the fourth such incident since November.
Reuters reported an American official saying it was the first joint military exercise by China and Russia since Moscow invaded Ukraine in February.
“We think it shows that China continues to be willing to closely align themselves with Russia, including through military cooperation,” the official said.
“China is not walking away from Russia.
“Instead, the exercise shows that China is ready to help Russia defend its east while Russia fights in its west.”
Russia and China both said they had conducted a joint patrol, with Moscow adding the operation lasted 13 hours over the Japanese and East China seas.
The exercise was part of an “annual military cooperation plan”, China’s defence ministry claimed.