US airlines should notify Chinese authorities before flying through China's new air-defence zone, the State Department said, even as the American military conducts daily flights in the area without such notification.
The United States said it expected US carriers to operate in line with so-called "notices to airmen" issued by foreign countries. However, it said the decision "[did] not indicate US government acceptance of China's requirements".
The advice is in contrast with that of America's close ally Japan, where the two major airlines have agreed with the Japanese government to fly through the zone without notifying China.
US Vice-President Joe Biden will visit China, Japan and South Korea this week, and senior US officials said he would try to ease tensions over the issue.
The Yomiuri newspaper reported that the United States and Japan would issue a joint statement demanding China scrap the air zone during Biden's visit to Tokyo. Biden will meet Japanese Prime Minister Shinzo Abe on Tuesday.
Japan also has asked the UN civil aviation agency, the International Civil Aviation Organisation, to consider responses to China's move, NHK television reported. The report said Japan's move during the agency's meeting in Montreal, Canada, were backed by South Korea and the US.
The US advisory to airlines emerged hours after the US military disclosed that it has been flying daily through the disputed area without providing notice to Beijing authorities.
That disclosure by a defence official indicates that US flight activity in the area, where China has sought to exert control, is more extensive than was previously known.
"It's very important the US 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 defence treaty with Japan, said Nicholas Burns, a former US undersecretary of state.
The Pentagon had acknowledged a flight by two unarmed B-52 bombers through the air zone on Tuesday. The defence official would not specify the type of aircraft used in subsequent flights, nor say whether any of them were armed.
Japan and South Korea also flew military aircraft through the area last week without informing Beijing. On Friday, China scrambled jets after two US spy planes and 10 Japanese aircraft, including F-15 fighters, entered the zone.
Defence Ministry spokesman Yang Yujun said it was "incorrect" to suggest China would shoot down aircraft which entered the zone without first identifying themselves.
Ties between China and Japan have been strained for months by a dispute over a group of islands, called the Diaoyus by China and the Senkakus by Japan.
"It's important for both sides to take a calm approach and deal with the situation according to international norms," Japanese Defence Minister Itsunori Onodera told state broadcaster NHK yesterday.
Onodera said the Japanese military had not noted any Chinese aircraft in the zone.
Reuters, Bloomberg, Agence France-Presse