Two American B-52 bombers flew over a disputed area of the East China Sea without informing Beijing, challenging China's claims to an expanded air defense zone, officials said Tuesday.
The flight of the giant, long-range Stratofortress planes sent a clear warning that Washington would push back against what it considers an aggressive stance by Beijing in the region.
The move also signaled staunch US support for Japan, which has been locked in a mounting feud with Beijing over disputed islands in the East China Sea.
The unarmed bombers took off from Guam on Monday on a scheduled flight, as part of what defense officials insisted was a routine exercise dubbed "Coral Lightning Global Power Training Sortie."
"Last night we conducted a training exercise that was long-planned. It involved two aircraft flying from Guam and returning to Guam," Pentagon spokesman Colonel Steven Warren told reporters.
Although China has insisted it has a right to police the skies over the area, no flight plan was submitted beforehand to the Chinese and the mission went ahead "without incident," Warren said.
The two aircraft spent "less than an hour" in China's unilaterally-declared Air Defense Identification Zone (ADIZ) and did not encounter Chinese planes, he said.
A defense official, speaking on condition of anonymity, confirmed to AFP the two US planes were B-52 bombers.
The military flight carried important symbolism as it came a week before US Vice President Joe Biden's scheduled trip to China, Japan and South Korea next month.
China announced the expanded air defense zone amid a sovereignty dispute with Japan over the island chain in the East China Sea known as Senkaku in Japanese and Diaoyu in Chinese.
The area also includes waters claimed by Taiwan and South Korea, which also have both denounced Beijing's move.
Without taking sides in the territorial argument, UN Secretary General Ban Ki-moon on Tuesday called on China and Japan to negotiate an end to their dispute.
Ban on Tuesday said tensions should be handled "amicably through dialogue and negotiations."
Under the rules declared by China, aircraft are expected to provide a flight plan, clearly mark their nationality and maintain two-way radio communication to allow them to respond to identification queries from Chinese authorities.
Pentagon officials said the United States views the area as international air space and American military aircraft would operate in the zone as before without submitting flight plans to China in advance.
Japan, the United States and several other governments promptly rejected China's announced air defense zone after it was announced Saturday.
The US State Department renewed its criticism Tuesday, saying China's action appeared to be an attempt to "unilaterally change the status quo in the East China Sea."
The move "will raise regional tensions and increase the risk of miscalculation, confrontation and accidents," State Department spokeswoman Jen Psaki said.
Australia summoned Beijing's ambassador to express its opposition and Japanese airlines said Wednesday they had stopped following China's new rules.
The reversal came after pressure from the Japanese government, which insisted China's announcement was invalid, and after governments around the world lined up alongside Tokyo.
France and Germany each expressed concern and urged restraint on all sides.
"We are committed to a peaceful resolution through dialogue to this dispute, in accordance with international law," French foreign ministry spokesman Romain Nadal said.
The territorial dispute over the islands has simmered for decades but in September 2012, Japan nationalized three of the islands, in what it portrayed as an attempt to avoid a more inflammatory step by a nationalist politician.
Beijing, however, accuses Tokyo of disturbing the status quo, and has sent ships and planes to the islands in a show of force.
In response, Japan has mobilized vessels and aircraft, raising fears the tensions could trigger an accidental clash.
Beyond the East China Sea, Beijing has taken an assertive approach to a number of territorial disputes, particularly in the strategic South China Sea.
In response to China's growing military might and influence, the United States has sought to shift its strategic focus to Asia, planning to expand America's military presence across the Pacific.
But a spate of crises in the Middle East and budget woes have often hampered Washington's attempt at a so-called "pivot" to the Asia-Pacific.