The United States has pledged support for ally Japan in a growing dispute with China over islands in the East China Sea.
US Defence Secretary Chuck Hagel assured his Japanese counterpart in a phone call that the two nations' defence pact covered the small islands where China established a new airspace defence zone last week.
A Pentagon spokesperson said Mr Hagel commended Tokyo "for exercising appropriate restraint."
China's declaration raised the stakes in a territorial stand-off between Beijing and Tokyo over the area, which includes the tiny uninhabited islands known as the Senkaku in Japan and the Diaoyu in China, Reuters reports.
The US defied China's demand that aeroplanes flying near the islands identify themselves to Chinese authorities, flying two unarmed B-52 bombers over the islands on Tuesday without informing Beijing.
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.
