(Reuters) – U.S. airlines United , American and Delta, have notified Chinese authorities of flight plans when traveling through an air defense zone Beijing has declared over the East China Sea, following U.S. government advice.

The zone has raised tensions, particularly with Japan and South Korea, and is likely to dominate the agenda of a visit to Asia this week of U.S. Vice President Joe Biden. He will travel to Japan, China, and South Korea and try to ease tensions, senior American officials said.

However, China’s declaration of the zone also represents a historic challenge by the emerging world power to the United States, which has dominated the region for decades.

China published co-ordinates for the zone last weekend. The area, about two-thirds the size of the United Kingdom, covers most of the East China Sea and the skies over a group of uninhabited islands at the center of a bitter territorial dispute between Beijing and Tokyo.

Beijing wants all foreign aircraft passing through the zone, including passenger planes, to identify themselves to Chinese authorities.

On Friday, the United States said it expected U.S. carriers to operate in line with so-called notices to airmen issued by foreign countries, although it added that the decision did “not indicate U.S. government acceptance of China’s requirements.”

A spokesman for Delta Airlines said it had been complying with the Chinese requests for flight plans for the past week. American and United said separately that they were complying, but did not say for how long they had been doing so.

Airline industry officials said the U.S. government generally expected U.S. carriers operating internationally to comply with notices issued by foreign countries.

In contrast, Japanese carriers ANA Holdings and Japan Airlines have flown through the zone without informing China, under an agreement with the Tokyo government. Neither airline has experienced problems.

The airlines said they were sticking with the policy even after Washington’s advice to its carriers.

Any sign that the United States was even tacitly giving a nod to China’s air defense zone would disturb Tokyo, which is hoping for a display of solidarity when Biden visits Japan starting on Monday.

“We will have in-depth talks about it,” Japanese Prime Minister Shinzo Abe was quoted as saying by Japan’s Kyodo news agency. “Japan and the United States will address it in close co-ordination with each other.”

However, he also insisted that the United States had not advised its airlines to comply with Chinese demands for prior notice before their planes enter the new air defense zone.

“We have confirmed through diplomatic channels that the U.S. government didn’t request commercial carriers to submit flight plans,” he was quoted as saying in the Kyodo report.

And Defense Minister Itsunori Onodera insisted the allies were working in lockstep.

“I believe the U.S. government is taking the same stance as the Japanese government,” he said in an interview with public broadcaster NHK.

Separately, Japan’s foreign affairs ministry said it had raised China’s declaration of the air defense zone with the International Civil Aviation Organization, the international aviation regulatory body and an agency of the United Nations.

It wasn’t immediately clear what Japan wanted the agency to do, since it can make no more than non-binding recommendations. But Japan’s action puts the issue before a global and multi-lateral body.

Since the zone came into force there has been no impact on the safe operation of international civilian flights, China’s Foreign Ministry said on Thursday. Still, China “hoped” airlines would co-operate, the ministry said.

The United States, Japan and South Korea have defied the Chinese move by flying military aircraft, including giant U.S. B-52 bombers, through the zone without informing Beijing.

A U.S. official said China’s action appeared to be a unilateral attempt to change the status quo in the East China Sea, which could “increase the risk of miscalculation, confrontation and accidents”.

“We urge the Chinese to exercise caution and restraint, and we are consulting with Japan and other affected parties throughout the region,” the official said.

China scrambled jets on Friday after two U.S. spy planes and 10 Japanese aircraft, including F-15 fighters, entered the zone, China’s state news agency Xinhua said. The jets were scrambled for effective monitoring, it quoted air force spokesman Shen Jinke as saying.

The Chinese patrol mission, conducted on Thursday, was “a defensive measure and in line with international common practices,” Shen said, according to Xinhua.

“China’s air force is on high alert and will take measures to deal with diverse air threats to firmly protect the security of the country’s airspace,” he said.

However, Defense Ministry spokesman Yang Yujun said it was “incorrect” to suggest China would shoot down aircraft which entered the zone without first identifying themselves.

U.S. flights were “routinely” transiting the zone, U.S. officials said on Friday.

“These flights are consistent with long standing and well known U.S. freedom of navigation policies,” Pentagon spokesman Colonel Steve Warren said. “I can confirm that the U.S. has and will continue to operate in the area as normal.”

A U.S. defense official said routine operations included reconnaissance and surveillance flights.

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