The United States pledged support for ally Japan in a growing dispute with China over islands in the East China Sea and senior US administration officials accused Beijing of behavior that had unsettled its neighbors.
US Defense Secretary Chuck Hagel assured his Japanese counterpart in a phone call that the two nations' defense pact covered the small islands where China established a new airspace defense zone last week and commended Tokyo for exercising appropriate restraint, a Pentagon spokesman said.
China's declaration raised the stakes in a territorial standoff between Beijing and Tokyo over the area, which includes the tiny uninhabited islands known as the Senkaku in Japan and the Diaoyu in China.
The United States defied China's demand that airplanes flying near the islands identify themselves to Chinese authorities, flying two unarmed B-52 bombers over the islands on Tuesday without informing Beijing.
It was a sharp reminder to China that the United States still maintains a large military presence in the region despite concerns among US allies that President Barack Obama's pivot to Asia strategy has borne little fruit.
In a previously announced trip, Vice President Joe Biden will visit China, Japan and South Korea next week. He will seek to ease tensions heightened by China's declaration, senior administration officials said.
Washington does not take a position on the sovereignty of the islands but recognizes that Tokyo has administrative control over them and the United States is therefore bound to defend Japan in the event of an armed conflict.
Some experts say the Chinese move was aimed at eroding Tokyo's claim to administrative control over the area.
China's Defense Ministry said it had monitored the US bombers. A Pentagon spokesman said the planes had not been observed or contacted by Chinese aircraft.