A group of disputed islands, Uotsuri island (top), Minamikojima (bottom) and Kitakojima, known as Senkaku in Japan and Diaoyu in China is seen in the East China Sea, in this photo taken by Kyodo September 2012. REUTERS/Kyodo

By Marina Lopes and Lesley Wroughton

NEW YORK/WASHINGTON (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 United States said on Friday 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 done so.

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

In contrast, two major airlines in Japan, the United States' close ally, have agreed with the Japanese government that they would fly through the zone without notifying China.

China published coordinates 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 dispute between Beijing and Tokyo.

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

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

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.

U.S. Vice President Joe Biden is due to visit China, Japan and South Korea next week, and will try to ease tensions over the issue, senior American officials 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.

STRAINED TIES

Japanese carriers ANA Holdings and Japan Airlines have flown through the zone without informing China. Neither airline has experienced problems.

The airlines said they were sticking with the policy even after Washington advised U.S. commercial airlines to notify China when they fly through the zone.

China's Foreign Ministry said on Thursday there had been no impact on the safe operation of international civilian flights since the zone came into force, although China "hoped" airlines would co-operate.

Ties between China and Japan have been strained for months by the dispute over the islands, called the Diaoyu by China and the Senkaku by Japan.

Mutual mistrust over military intentions and what China feels is Japan's lack of contrition over its brutal occupation of parts of China before and during World War Two have added to tension.

Story continues