The coordinated bomber flights were the first such activity the two nations had done together since President Vladimir V. Putin of Russia ordered the invasion of Ukraine.
WASHINGTON — China and Russia on Tuesday held their first joint military exercise since Moscow’s invasion of Ukraine, sending bombers over the seas in northeast Asia as a show of force as President Biden was visiting the region, according to American and South Korean officials.
The U.S. government was tracking the military exercise, a senior American official said, as Mr. Biden met in Tokyo with the leaders of Australia, Japan and India, partner nations in the so-called Quad coalition that was formed in part to counter Chinese power in the Indo-Pacific region. The military activity was a significant sign that the partnership between China and Russia has not weakened even as the three-month-old war in Ukraine has resulted in thousands of civilian deaths.
The bombers flew over the Sea of Japan early Tuesday and continued south toward the East China Sea and the Philippine Sea, the U.S. official said, speaking on the condition of anonymity. South Korea issued a statement confirming the exercise, saying that two Chinese military aircraft and four Russian warplanes had entered its air defense identification zone off the country’s east coast, without intruding into its air space.
Joint exercises involving strategic bombers are complex and are typically planned well in advance. The official declined to give immediate details of the number of bombers involved or how the United States was tracking the exercise.
President Vladimir V. Putin has tried to strengthen Russia’s ties with China as a like-minded nation standing in opposition to Western dominance. On Feb. 4, as Mr. Putin visited Beijing for the Winter Olympics and met with President Xi Jinping for the 38th time as national leaders, their governments issued a 5,000-word statement that declared the two nations had a “no limits” partnership.
Senior U.S. officials and a European official later said in interviews that a Western intelligence report had indicated that senior Chinese officials asked their Russian counterparts in early February to hold off on invading Ukraine until after the Olympics ended. The day after the closing ceremony, Mr. Putin declared that Ukraine should not be a sovereign state and ordered more units of the Russian military to cross into the embattled Donbas region of eastern Ukraine. The full-scale invasion began three days later.
In mid-March, U.S. officials said Russia had asked China for military and economic aid after the invasion of Ukraine. Russian ground forces have fared poorly in battles and skirmishes against the Ukrainian military, and Mr. Putin has decided to withdraw units from around Kyiv and other major cities and concentrate on taking the entire Donbas region. American officials say they have not detected any military or economic aid sent from China to Russia for the war.
China has also not stepped in to help Russia evade sanctions or blunt the impact of the penalties, U.S. officials say.
Chinese and Russian officials have been strengthening their military ties in recent years, and the two nations have been growing closer in part because of the highly personal bond between Mr. Xi and Mr. Putin. Both men are autocrats who harbor hostility toward the United States and aim to weaken American power.
China has been buying more advanced weapons from Russia, and the two nations have done a growing number of joint military exercises recently. Last October, the two countries held joint naval drills off the Russian Far East. This January, the two joined with Iran for the same type of exercises in the northern Indian Ocean.
The United States has been monitoring the rapid modernization of the People’s Liberation Army of China and its various forces. Russia’s invasion of Ukraine has provoked greater anxiety among some U.S. and European officials over the possibility that Beijing might decide to invade Taiwan. U.S. officials have been pushing Taiwan to order American-made weapons that they say would give Taiwan a fighting chance against a typical seaborne invasion.
On Monday, Mr. Biden said the United States would defend Taiwan militarily if China attacked the self-governing democratic island. U.S. officials said later that Mr. Biden was not changing a longstanding policy of “strategic ambiguity” on Taiwan and China in any way.