BEIJING (AFP) - China insisted on Wednesday it has the capacity to enforce its controversial newly-declared air zone over islands disputed with Japan, despite Beijing's reluctance to intervene after American B-52 bombers flouted its rules.

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

Beijing's non-confrontational response elicited scorn from some Chinese netizens as weak in the face of defiance, but analysts said it may never have intended to impose the zone by force.

"The Chinese government has the will and ability to defend our national sovereignty and security," foreign ministry spokesman Qin Gang said at a regular press briefing.

"We also have the ability to exercise effective control over the East Sea Air Defence 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.

China's demand 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."

The statement did not include any expression of regret or anger.

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

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

Japan, the United States and several other governments condemned the zone after it was announced over the weekend, and the US State Department reiterated Tuesday that it appeared to be an attempt to "unilaterally change the status quo in the East China Sea".

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

Its ambassador to Tokyo Caroline Kennedy said on Wednesday: "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 on Wednesday after initially complying.

The bombers - which were unarmed - took off from Guam on Monday on a scheduled flight in what US defence officials insisted 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.

"Not daring to shoot it down shows that establishing an ADIZ is meaningless," wrote another.

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 international security expert Jingdong Yuan at the University of Sydney.

Mr Gary Li, a senior fellow at consultancy IHS Maritime, said: "You don't necessarily have to respond to everything and have that necessarily mean you look weak if you don't.

"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".

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

Beyond the East China Sea, Beijing has taken an assertive approach to territorial disputes in the South China Sea, while the US has sought to shift its strategic focus to Asia in response to China's growing might.