The United States is advising all US airlines to take steps to stay safe amid fresh tensions in the East China Sea as they seek clarification about Beijing's newly-declared air zone over disputed islands.

"We're attempting to determine whether the new rules apply to civil aviation, commercial air flight," State Department spokeswoman Jen Psaki said.

"In the meantime, US air carriers are being advised to take all steps they consider necessary to operate safely in the East China Sea region."

But she would not be drawn on whether US officials were advising commercial airlines to advise Chinese authorities of any flight plans which take them over the islands.

"We're still looking into and evaluating ... what this means, and obviously, we want to advise accordingly."

China demanded over the weekend that aircraft submit flight plans when traversing its newly-declared air defense identification zone (ADIZ) which includes the disputed islands, known as Senkaku in Tokyo and Diayou in Beijing.

The move has triggered a storm of diplomatic protest and in a show of force two American B-52 bombers overflew the area on Monday.

In a statement, Chinese defense ministry spokesman Geng Yansheng said: "The Chinese military monitored the entire process, carried out identification in a timely manner, and ascertained the type of US aircraft."

The Chinese ADIZ requires aircraft to provide their flight plan, declare their nationality and maintain two-way radio communication, or face defensive emergency measures.

Japanese airlines, under pressure from Tokyo, stopped following China's new rules Wednesday, after initially complying.