The United States says it will continue surveillance flights over a disputed area of the East China Sea after Beijing announced it had scrambled fighter jets to trail American and Japanese military aircraft.

China recently declared an air defence identification zone (ADIZ) in the area, which includes islands at the heart of a territorial dispute with Japan.

Japan and South Korea both said on Thursday they had disregarded the ADIZ, showing a united front after US B-52 bombers also entered the area.

Tokyo, Seoul and Washington have accused China of increasing regional tensions with its air defence zone declaration.

A Foreign Ministry spokesman in Beijing, Qin Gang, says China is simply defending itself.

"China's establishment of the zone was not directed towards any specific country or target. It's entirely for the purpose of exercising China's right of self defence and will not cause regional tensions," Qin said.

Chinese state-run media earlier identified Japan as Beijing's "prime target" in the air defence zone, calling for "timely countermeasures without hesitation" if Tokyo defies it.

Other countries, however, which have sent military aircraft into the air defence identification zone (ADIZ), including the United States and South Korea, should be largely ignored, the Global Times said.

"If the US does not go too far, we will not target it in safeguarding our air defence zone."

Washington says US planes will continue to enter the zone.

"We have flights routinely transiting international airspace throughout the Pacific, including the area China is including in their ADIZ," said Pentagon spokesman Colonel Steve Warren.

"These flights are consistent with long-standing and well-known US freedom of navigation policies that are applied in many areas of operation around the world. I can confirm that the US has and will continue to operate in the area as normal."

The US State Department said on Friday that American commercial airlines should observe China's demand to be given notice of planes entering the air defence zone.

Compliance by commercial flights "does not indicate US government acceptance of China's requirements for operating in the newly declared ADIZ," the State Department said in a statement.

Aircraft patrols escalate tension

Map China's new "air defence identification zone", published by state-run media Xinhua News Agency

China's ADIZ, which was declared on Saturday, requires aircraft to provide their flight plan, declare their nationality and maintain two-way radio communication, or face military intervention.

The zone overlaps area declared by Japan and South Korea, and includes airspace over a group of uninhabited islands controlled by Japan but claimed by China.

The islands are called the Diaoyu in China and the Senkaku in Japan.

ABC/AFP