Russian and Chinese military planes have flown joint patrols near Japanese and South Korea air defence zones in a pointed farewell to US President Joe Biden as he concludes a trip to Asia that has rankled officials in Beijing.
The patrols came hours after Biden angered China by saying he would be willing to respond militarily to defend Taiwan if it came under Chinese attack, and as he discussed responses to Russia's invasion of Ukraine with leaders of the Quad, which groups the United States with Australia, India and Japan.
Japan said it scrambled jets after Russian and Chinese warplanes neared its airspace while Tokyo was hosting the Quad leaders.
Japan called the drills a provocation.
It was the first joint military exercise by China and Russia since the invasion of Ukraine on February 24, according to a senior US official, and it came at the end of Biden's four-day trip to South Korea and Japan.
"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, adding that such actions must be planned well in advance.
China and Russia declared a "no-limits" partnership just weeks before the invasion, which China has refused to condemn.
However, US officials say they have yet to see signs that China is directly supporting Russia's war effort, or breaking US-led sanctions.
Biden's trip was part of US efforts to push back against what it calls China's "coercive" behaviour, including against Taiwan, a self-ruled island China claims as it own.
In Asia, Biden vowed that the US would stand with allies and partners to push for a free and open Indo-Pacific region.
At a news conference on Monday, Biden said "yes" when asked if he would be willing to respond militarily to a Chinese attack on Taiwan, seeming to break with a long-held policy of not making clear how the United States might react.
It was the latest in a series of apparently off-the-cuff assertions that suggest his personal inclination is to defend the island, although Biden said US policy had not changed.
Some critics have said he has mis-spoken or made a gaffe.
But other analysts have suggested that given Biden's extensive foreign policy experience and the context - he was speaking next to the Japanese prime minister and while the war in Ukraine raged on - he had not spoken in error.
Analysts and advisers said Biden went to Asia with a clear message for China not to try what Russia did in Ukraine anywhere in the region, especially not Taiwan.
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 bombers, Russia's defence ministry said.
Japanese and South Korean air force planes shadowed the Russian and Chinese aircraft for part of the exercise, it said.
China's defence ministry said it was part of an annual military exercise.
A US official said Chinese naval vessels likely participated in the drills.
Japan conveyed "grave concerns" to both Russia and China through diplomatic channels, its Defence Minister Nobuo Kishi told a news conference.
"We believe the fact that this action was taken during the Quad summit makes it more provocative than in the past," he said.
The Quad meeting brought Biden together with Indian Prime Minister Narendra Modi, Australia's newly elected leader Anthony Albanese and Japanese Prime Minister Fumio Kishida.
In a joint statement, the leaders said they "discussed their respective responses to the conflict in Ukraine and the ongoing tragic humanitarian crisis".
But in an apparent concession to India, which maintains close ties with Russia, the words "Russia" or "Russian" did not appear.
Australian Associated Press
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