TOKYO—The U.S. and Japan moved swiftly to warn China to refrain from escalating territorial tensions in the East China Sea, after Beijing stepped up the ante in its dispute with Tokyo by declaring a new air defense zone covering the islands claimed by both nations.
Hours after China unveiled Saturday its new air defense identification zone, or ADIZ, and warned of military action against foreign aircraft that don't cooperate in its enforcement, the U.S. took an unusual step of issuing statements by Secretary of State John Kerry and Secretary of Defense Chuck Hagel, in which they sharply criticized China's action and reiterated Washington's commitments to defend Japan in case of a conflict.
Saturday's announcement from Beijing heralded a potentially dangerous new phase of the territorial dispute as it created an extensive area of overlap between Chinese and Japanese air defense zones. Xinhua published Saturday the coordinates of the zone and a map showing that it covers the disputed islands and at its easternmost point stretches to within about 130 kilometers (81 miles) from Kyushu, one of Japan's main islands.

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Since the island dispute flared up over a year ago, U.S. officials have expressed concerns that an unintended clash or a collision in the contentious areas between ships or airplanes from the two countries could escalate into a military confrontation and drag the U.S. into an unwanted feud.
Underscoring such worries, Japan said Saturday it scrambled fighter jets against two Chinese military aircraft after they entered Japan's own ADIZ and approached the uninhabited islands known as Senkaku in Japan and Diaoyu in China. The emergency dispatch of the Japanese jets came after China said it conducted the first patrol of its new air defense zone using two large "scout" aircraft backed by fighter jets and early warning planes, according to the Xinhua news agency.
Japan's defense ministry said one of the two Chinese planes, a TU-154 passenger-style aircraft now used for military information gathering, flew to the area north of Senkaku but didn't enter Japan's airspace. Tokyo had described China's new ADIZ as "totally unacceptable" and lodged protests through diplomatic channels.
The new Chinese zone isn't likely to deter Japan or the U.S. from continuing military operations in the area, according to several analysts.
The big question, those analysts said, however, is how China responds to Japanese and U.S. aircraft entering the new ADIZ. In the past, China has generally only intercepted foreign military aircraft on the fringes of its national airspace, which extends 12 nautical miles from its shores.
"We'll have to see what happens the next time Japan sends up its F-15s," said James Hardy, Asia-Pacific editor for IHS Jane's Defence Weekly.
"That's why people are quite scared, because you're waiting for events on the ground to create a policy around it. That's not a good way to resolve international problems because at the operational level, people make mistakes. When you get fast jets involved, the margin for error is much smaller and the potential for lethality is much greater."
Foreign military officials and analysts say the Chinese military has little experience of conducting interceptions further from its territory, and has yet to declare clear guidelines for how its pilots should act.
"It's a high altitude game of chicken," said Carlyle Thayer, an expert on Asian maritime security issues at the Australian Defence Force Academy.
The swift and strongly-worded statements from Messrs. Kerry and Hagel indicate Washington's growing concern over the escalation in the territorial dispute between China, with its soaring military and economic prowess, and Japan, one of Washington's key allies.
"We view this development as a destabilizing attempt to alter the status quo in the region. This unilateral action increases the misunderstanding and miscalculations," Mr. Hagel said. He reiterated that the U.S.'s obligation under its bilateral security treaty to defend Japan when its territory comes under armed attack "applies to the Senkaku Islands."
Calling China's latest action "escalatory," Mr. Kerry noted the U.S. has urged China to "exercise caution and restraint."
Over the past year, China has increasingly asserted its claim over the islands controlled by Tokyo by frequently dispatching its patrol ships and planes, setting off cat-and-mouse chases with their Japanese counterparts. There have been a few incidents where the two sides came into dangerously proximity, including locking of weapons-targeting radar by Chinese ships onto Japanese military targets, and scrambling of Chinese fighter jets in response to the dispatch of Japanese military planes.
Japan is one of many countries with an ADIZ, within which it reserves the right to identify, communicate with and intercept approaching foreign aircraft. Japan's zone extends 200 nautical miles from its shore, well beyond its national airspace and reaching close to China's coastal areas.
Even as it reacted to the Chinese planes' flight through its ADIZ by scrambling fighters, Tokyo's response to the latest provocation has been restrained. The protests to the Chinese government were conveyed by working-level officials below the top ranks, a move one foreign ministry official attributed to Japan's stance of "not initiating the escalation of tensions on our side."
Aside from a written statement of protest from the foreign ministry, no formal statements were made by its top officials including Prime Minister Shinzo Abe and Foreign Minister Fumio Kishida. Defense Minister Itsunori Onodera gave brief remarks to reporters but refrained from using confrontational language. "It is very important to avoid any kind of clash. We'd like to urge China to set up a communication mechanism so we can respond in case of an emergency," he said. Mr. Onodera added Japan would step up patrol and surveillance activities in the areas surrounding the Senkakus.
The establishment of the new ADIZ could represent a setback in the bilateral relations just as foreign policy experts began to see signs of a thaw for the first time since the flare-up in the long-standing dispute triggered by the nationalization of the disputed islands by Japan in September last year. The news came as a group of 178 executives from top Japanese companies was wrapping up a visit to China as the largest business mission between the two nations in over two years. The number of Chinese tourists visiting Japan is finally rebounding and bilateral trade volume has been on the rise.
China's announcement also prompted expressions of concern from Taiwan—the island China regards as a rebel province—and from South Korea, both of which have ADIZs that overlap with the new Chinese one.
Taiwan's government was "highly concerned", said a joint statement from its ministries of defense of foreign affairs and the Mainland Affairs Council, which handles relations with Beijing.
A spokesman for South Korea's defense ministry issued a statement saying the government in Seoul "regrets" that China's zone overlaps with its South Korean equivalent in an area off South Korea's southwest coast.
Some analysts said they expected China to set up a similar ADIZ over the South China Sea, where China's claims overlap with those of Vietnam, Malaysia, Taiwan, Brunei and the Philippines.
But they said they didn't expect that to happen soon as China didn't yet have the air power to enforce ADIZ over the huge area it claims, which stretches almost to the coast of Malaysia and the Philippines.
Write to Yuka Hayashi at yuka.hayashi@wsj.com and Jeremy Page atjeremy.page@wsj.com