Japan scrambled jets after Russian and Chinese warplanes neared its airspace while Tokyo was hosting the leaders of the Quad group of countries, which includes the United States, said Defense Minister Nobuo Kishi, who called the move a provocation.
It was the first joint military exercise by China and Russia since Moscow invaded Ukraine on February 24, according to a US official, and it came at the tail end of Biden's four-day trip.
"We think it shows that China continues to be willing to closely align themselves with Russia, including through military cooperation," a senior administration official said, adding that such actions must be planned well in advance.
"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," the official said.
Biden stressed during the trip, intended in part to counter China's growing influence in the region, that the United States will stand with its allies and partners to push for a free and open Indo-Pacific region.
Beijing and Moscow declared a "no-limits" partnership just weeks before Russian President Vladimir Putin ordered the attack on Ukraine.
The joint patrol lasted 13 hours over the Japanese and East China seas and involved Russian Tu-95 strategic bombers and Chinese Xian H-6 jets, the Russian defense ministry said in a statement.
Planes from the Japanese and South Korean air forces shadowed the Russian and Chinese jets for part of the exercise, it said.
Tokyo conveyed "grave concerns" to both Russia and China through diplomatic channels, Kishi said at a news conference.
He characterized the incident as a likely provocation by both Beijing and Moscow on a day when Biden, Indian Prime Minister Narendra Modi and Australia's newly elected leader, Anthony Albanese, were meeting in Tokyo.
"We believe the fact that this action was taken during the Quad summit makes it more provocative than in the past," he said, adding it was the fourth such incident since November.
Chinese naval vessels likely participated in the joint exercise, a US official said.
China's defense ministry confirmed the joint aerial patrol over the Sea of Japan, East China Sea and the Western Pacific and called it part of an annual military exercise.
On Monday, Biden angered China by saying he would be willing to use force to defend Taiwan, but he said later US policy toward the self-ruled island had not changed. Beijing considers Taiwan an inalienable part of China that should be reunited with the mainland. (Reuters)