Russian and Chinese military planes conducted joint exercises to patrol the Asia-Pacific region, Russia's defense ministry said on Tuesday, in a pointed farewell to U.S. President Joe Biden as he concluded an Asia trip that rankled Beijing.

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 defense ministry said in a statement.

Japan's defense minister called the exercise a provocation, while a U.S. official said it revealed the depth of the two countries' cooperation.

Planes from the Japanese and South Korean air force shadowed the Russian and Chinese jets for part of the exercise, the Russian ministry said.

Japan scrambled jets after Russian and Chinese warplanes neared its airspace while Tokyo was hosting the leaders of the Quad grouping of countries that includes the United States, Defence Minister Nobuo Kishi said.

Tokyo conveyed "grave concerns" to both Russia and China through diplomatic channels, Kishi said at a news conference.

He characterised 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.

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.

The move marks the first joint military exercise by China and Russia since Moscow invaded Ukraine on Feb. 24, according to a U.S. official, and it came at the tail end of Biden's trip.

A joint strategic bomber exercise shows the depth of the two countries' alignment, a senior U.S. administration official said.

"We think it shows that China continues to be willing to closely align themselves with Russia, including through military cooperation," the 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.

On Monday, Biden angered China by saying he would be willing to use force to defend Taiwan, but he said later U.S. policy toward the self-ruled democratic island had not changed. China considers Taiwan an inalienable part of its territory that should be reunited with the mainland.

Tuesday's incursion was the first reported since new South Korean President Yoon Suk-yeol took office on May 10. On Sunday, Yoon wrapped up his summit with Biden, where the two leaders pledged support for measures seen as countering China's influence in the region, and criticized Russia's war in Ukraine.

South Korea's military said it scrambled fighter jets after at least four Chinese and four Russian warplanes entered its air defense zone.

The Russian and Chinese aircraft entered and left the Korea Air Defence Identification Zone (Korea ADIZ) in the Sea of Japan, known in Korea as the East Sea, several times through the day, according to South Korea's Joint Chiefs of Staff.

The aircraft, which included fighter jets and bombers from each side, did not violate South Korea's airspace, it said.

South Korea had no warning of the apparent drills, a military source in Seoul said. When Seoul saw that the aircraft appeared to be headed toward the defense zone, it used hotlines to warn Chinese and Russian counterparts, the source said.

China responded that it was a regular exercise, the source added, while there was no response from Russia.

Unlike airspace, an air defense identification zone is usually an area where countries may unilaterally demand that foreign aircraft take special steps to identify themselves, with no international laws governing ADIZs.