China has insisted it has the ability to enforce its newly-declared air zone over islands disputed with Japan, despite Beijing's reluctance to intervene after American B-52 bombers entered the area.

The flight of the giant long-range US Stratofortress planes was a clear warning that Washington would push back against what it considers an aggressive stance.

While US defence chief Chuck Hagel praised Tokyo's restraint, officials indicated Vice President Joe Biden would personally convey America's "concerns" about the matter during a visit to the Chinese capital next week.

Qin Gang, the foreign ministry spokesman in Beijing told reporters Wednesday: "The Chinese government has the will and ability to defend our national sovereignty and security."

"We also have the ability to exercise effective control over the East Sea Air Defense Identification Zone," (ADIZ) he said.

The area in the East China Sea includes Japan-administered islands at the heart of a tense dispute between the two neighbours, known as Senkaku in Tokyo and Diaoyu in Beijing.

A Chinese demand over the weekend that aircraft submit flight plans when traversing it triggered a storm of diplomatic protest and the Pentagon said the B-52s did not comply.

But in a statement, Chinese defence 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."

Biden, scheduled to meet with Chinese President Xi Jinping and other high-ranking officials during his visit, was poised to address the matter head-on.

"Clearly, the visit to China creates an opportunity for the vice president to discuss directly with policymakers in Beijing this issue, to convey our concerns directly and to seek clarity regarding the Chinese intentions in making this move at this time," a senior US administration official told reporters.

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

The manoeuvres have raised fears of an accidental clash but analysts stress that both sides have commercial incentives to avoid conflict.

State-run media say it extends as close to Japan as Tokyo's zone approaches China.

The B-52 flight was also a signal of US support for Japan, with which Washington has a security pact.

The American ambassador to Tokyo, Caroline Kennedy, said: "The Japanese can see every day that America is here for them as a partner in the defence of Japan."

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

The US bombers -- which were unarmed -- took off from Guam on Monday on a scheduled flight in what American defence officials insist was a routine exercise.

Users of China's Twitter-like Sina Weibo accused their government of buckling when challenged.

"They came to test us and proved you don't have the guts to show them who's boss," said one.

But analysts said Beijing -- where a key Communist Party meeting took place earlier this month -- had remained vague about how it might enforce its authority and may never have intended to react in the field.

It may have simply wanted to declare an ADIZ to match Japan's and further assert its claim to the contested islands, they said.

Beijing left its options open "so they can explain away things like why there's nothing they can do about the violation of their ADIZ", said Jingdong Yuan, an international security expert at the University of Sydney.

Gary Li, a senior fellow at consultancy IHS Maritime, said the ADIZ "is entirely designed to give the Chinese more options on the diplomatic side of the argument, give them more tools, more leverage."

Chinese officials and state media have accused Japan and the US -- which both have ADIZs -- of double standards, and argue that the real provocateur is Tokyo.

The islands dispute, which has simmered for decades, escalated in September 2012 when Japan purchased three of the uninhabited outcrops from private owners.

Beijing accused Tokyo of changing the status quo and has since sent ships and planes to the area as displays of force, prompting Japan to scramble fighter jets 386 times in the year to September.

After an unidentified drone flew towards the islands, Tokyo threatened to shoot down such aircraft, which Beijing warned would amount to an "act of war".