U.S. airlines should notify Chinese authorities before flying through China’s new air-defense zone, the State Department said, even as the American military conducts daily flights in the area without such notification.

While U.S.-based carriers should meet China’s requirements for flying through the area, that recommendation doesn’t mean the American government accepts China’s assertion of the zone, the State Department said in a statement Thursday night.

"Freedom of overflight and other internationally lawful uses of sea and airspace are essential to prosperity, stability, and security," the agency said, repeating U.S. concern over China’s assertion of the air-defense identification zone.

The advisory to carriers emerged hours after the U.S. military disclosed that it has been flying daily through the disputed area without providing notice to Beijing authorities. That disclosure Thursday by a U.S. defense official indicates that U.S. flight activity in the area, where China has sought to exert control, is more extensive than was previously known.

"It’s very important the U.S. signal to the Chinese that we’re not going to be bullied and that we’re going to adhere to our commitments," which include a defense treaty with Japan, said Nicholas Burns, a former U.S. undersecretary of state for political affairs from 2005 to 2008.

The Pentagon had acknowledged a flight by two unarmed B-52 bombers through the air zone earlier this week. The defense official, who asked not to be named discussing military operations, wouldn’t specify the type of aircraft used in subsequent flights nor say whether any of them are armed.

‘Crisis’ Potential

China for a second day Thursday sent fighter planes into the air zone over an area that includes islands claimed by both China and Japan. The situation "holds the real potential for a crisis," said Dean Cheng, a senior research fellow at the Asian Studies Center of the Heritage Foundation in Washington.

China’s assertion and the subsequent flights have increased tensions that U.S. Vice President Joe Biden will seek to defuse when he visits Japan, China and South Korea next week.

Biden will convey U.S. concerns about China’s air zone and seek clarification from Chinese leaders on their intentions, according to an administration official who briefed reporters on condition of anonymity because the talks will be private.

China’s "provocative" behavior toward its neighbors in the region "now becomes a very prominent issue for the visit," said Burns, who’s now a professor of international relations at Harvard University’s Belfer Center for Science and International Affairs in Cambridge, Massachusetts.

‘Calming Role’

Biden is "well-positioned to play a calming role hopefully to defuse this crisis, in a way that is supportive of our primary friend, the government of Japan," Burns said in an interview.

China announced the air-defense identification zone effective Nov. 23 and said its military will take "defensive emergency measures" if aircraft enter the area without reporting flight plans or otherwise identifying themselves.

Japan, which denounced the move, told its airlines to stop providing flight plans to China. Japan and South Korea also have flown military aircraft through the air zone in recent days, testing China’s resolve to control a swath of the East China Sea that is central to a territorial dispute.

Japan and China both claim sovereignty over islands known as Diaoyu in Chinese and Senkaku in Japanese. The surrounding waters are rich in oil, natural gas and fish.

‘Destabilizing Attempt’

Defense Secretary Chuck Hagel has called China’s air zone "a destabilizing attempt to alter the status quo in the region" and warned that it "increases the risk of misunderstanding and miscalculations."

Richard Haass, president of the Council on Foreign Relations in New York, wrote a Twitter posting warning of the "real chance" of a military incident involving China, Japan and others over the air zone. Haass, a former State Department official, said the "challenge will be to manage" the situation and avoid an escalation.

While the U.S. is probably responding correctly, the military flights pose the danger of a conflict, most likely by accident if warning shots are fired that could be misinterpreted, Cheng said in an interview.

"It is a multilateral powder keg," he said. "You’re talking about something that will inevitably spill over to broader diplomatic relations and economic relations."

Pentagon Flights

The U.S. defense official said the Pentagon’s flights through China’s air zone are consistent with U.S. freedom-of- navigation policies that are applied to many areas of operation around the world.

Chinese planes were deployed in the zone off the country’s eastern coast and identified Japanese aircraft in the area, China’s Defense Ministry said in a statement on its website. Chinese planes entered the area Nov. 28 on normal patrols, the official Xinhua News Agency said.

The Chinese Defense Ministry statement also said its planes identified U.S. aircraft, without specifying whether they entered the air-defense zone.

Japanese Prime Minister Shinzo Abe said yesterday he would respond to China’s air zone in a "calm, assured manner."

Japan and the U.S. plan to step up air surveillance in the East China Sea, with Japan stationing E-2C airborne early- warning aircraft at the Naha base in the Okinawa region and expanding the use of unmanned Global Hawk aircraft, the Yomiuri newspaper reported Thursday, without citing a source.

‘Serious Challenges’

South Korea is considering expanding its own air-defense zone in response to China’s move, Wee Yong Sub, spokesman for the Defense Ministry said Thursday in Seoul.

"This is one of the most serious challenges ever posed by China to freedom of movement both on the sea and in the sky and will affect very seriously the forward deployment of the United States," Tomohiko Taniguchi, an adviser to Japan’s Abe, said in an interview with Bloomberg Television.

The State Department earlier this week recommended that U.S. carriers take all steps they consider necessary for safe operation in the zone. United Continental Holdings Inc., with the most service to China among U.S. carriers, will continue its usual policy of sharing flight data with China, according to Megan McCarthy, a spokeswoman for the Chicago-based airline.

"We are filing our flight plans with China as we normally do," she said in an e-mail earlier this week.

Japan’s major airlines, including ANA Holdings Inc. and Japan Airlines Co., have been flying through the zone without coordinating with China, all without incident.

Hagel called Japanese Defense Minister Itsunori Onodera earlier this week to assure the U.S. ally of its support. Hagel praised Japan "for exercising appropriate restraint" and "pledged to consult closely with Japan on efforts to avoid unintended incidents," according to a Pentagon statement.