Japan has received support from its US ally in a growing dispute with China over islands in the East China Sea after Beijing claimed airspace.
Chuck Hagel, the US defence secretary, told his Japanese counterpart in a telephone call on Wednesday that the two nations' defence treaty covers the small island group where China established a new airspace defence zone last week.
Hagel "commended the Japanese government for exercising appropriate restraint", a Pentagon spokesman said.
China's declaration raised the stakes in a territorial standoff between Beijing and Tokyo over the contested islands.
Some experts say the Chinese move was aimed at eroding Tokyo's claim to administrative control over the area, including the tiny uninhabited islands known as the Senkaku in Japan and the Diaoyu in China.
Washington does not take a position on the sovereignty of the islands but recognises that Tokyo has administrative control over them and the US is therefore bound to defend Japan in the event of an armed conflict.
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.
China's defense ministry said it had monitored the US bombers on Tuesday. A Pentagon spokesman said the planes had not been observed or contacted by Chinese aircraft.
The US signaled that more military flights into the defence zone claimed by China can be expected.
US Vice President Joe Biden is due to visit China, Japan and South Korea during a week-long trip and will seek to ease tensions heightened after China demanded that airplanes flying near the islands identify themselves to Chinese authorities, senior US administration officials said.
"The visit to China creates an opportunity for the vice president to discuss directly with policy makers in Beijing this issue to convey our concerns directly and to seek clarity regarding Chinese intentions," a senior administration official told reporters.
"It also allows the vice president to make the broader point that there's an emerging pattern of behaviour that is unsettling to China's own neighbours," the official said.
Biden will not be making a demand on a specific issue, but rather will raise the topic as part of talks spanning a range of themes, the official added.