The United States pledged support for ally Japan on Wednesday in a growing dispute with China over islands in the East China Sea as senior U.S. administration officials said China's claim to air space over the islands had unsettled its neighbors.
U.S. Defense Secretary Chuck Hagel told his Japanese counterpart in a phone call that the two nations' defense treaty covers the small island group where China established a new airspace defense zone last week and "commended the Japanese government for exercising appropriate restraint," a Pentagon spokesman said.
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U.S. 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 U.S. administration officials said.
China's declaration raised the stakes in a territorial standoff between Beijing and Tokyo over the contested islands.
The United States defied China's demand on Tuesday by flying two unarmed B-52 bombers over the islands without informing Beijing. Flights of Japan's main airline similarly ignored Chinese authorities while flying through that air space.
China's defense ministry said it had monitored the U.S. bombers. A Pentagon spokesman said the planes had not be observed or contacted by Chinese aircraft.
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 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.
Biden will seek to ease the growing tensions in the region during his trip next week, U.S. 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 behavior that is unsettling to China's own neighbors," the official said.
The official said it raised questions about "how China operates in international space and how China deals with areas of disagreement with its neighbors."
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.
The Pentagon signaled that more military flights into the defense zone claimed by China can be expected.
Colonel Steve Warren, a Pentagon spokesman, told Reuters: "We'll continue to conduct operations in the region, as we have" in the past. He declined to offer details on timing.
Japan Airlines and ANA Holdings said they had stopped giving flight plans and other information to Chinese authorities following a request from the Japanese government.
Both said they had not experienced any problems when passing through the zone. Japan's aviation industry association said it had concluded there was no threat to passenger safety by ignoring the Chinese demands, JAL said.