HONG KONG/BEIJING (REUTERS) - China's military could struggle to cope with the demands for intensified surveillance and interception if it tries to enforce the rules in its new air defence zone over islands at the heart of a territorial dispute with Japan.

Regional military analysts and diplomats said China's network of air defence radars, surveillance planes and fighter jets would be stretched by extensive patrols across its Air Defence Identification Zone, roughly two-thirds the size of Britain.

But some noted that even limited action could still spark alarm across a nervous region - and serve China's desire to pressure Japan.

China published the coordinates of its zone in the East China Sea over the weekend and warned it would take "defensive emergency measures" against aircraft that failed to identify themselves properly in the airspace.

It is already being tested.

Two unarmed US B-52 bombers on a training mission flew over the disputed islands on Monday without informing Beijing while Japan's main commercial airlines ignored the rules when their planes passed through the airspace on Wednesday.

China's Defence Ministry said it had monitored the entire progress of the US bombers. The Pentagon said the planes had neither been observed nor contacted by Chinese aircraft.

A Japanese government source said China's military, while growing rapidly after many years of double-digit budget increases, still did not have the radars or fighters to cover a zone of such size across international airspace.

"China will not implement (the zone) fully because they do not have enough assets... but they will try to scare smaller nations," said the source, who declined to be identified because he was not authorised to speak to the media on the topic.

While China could field an extensive array of surveillance capabilities, including ship-borne radar, there will still be gaps, added Mr Christian Le Miere, an East Asia military specialist at the independent International Institute of Strategic Studies in London.

"It is just not yet clear how they are going to enforce it," he said. "It may be more a rhetorical position to serve a political end."

China's creation of the zone triggered a storm of criticism from Washington and Tokyo, with both countries accusing Beijing of trying to change the status quo in the region.

Some experts have said the move was aimed at chipping away at Tokyo's claim to administrative control over the area, including the tiny uninhabited islands known as the Senkakus in Japan and the Diaoyu in China.

Japan and the United States have their own air defence zones but only require aircraft to file flight plans and identify themselves if those planes intend to pass through national airspace.

Mr Gary Li, a Beijing-based senior analyst with the consulting group IHS Aerospace, Defence and Maritime, said he did not believe China would try to replicate in the air what it had done at sea by keeping a rotating presence of coastguard ships on standby near the islands.

"I think it will be more a case of China flying enough to make a point - it is quite a strain on any force to maintain some kind of 24-hour presence in the air," he said.

"It must be remembered that this is not a no-fly zone - China doesn't have to operate extensive patrols to make its presence felt."

Patrol ships from China and Japan have been shadowing each other near the islets on and off for months, raising fears that a confrontation could develop into a clash.

There have also been several incidents involving military aircraft flying close to each other. In October, Chinese military aircraft flew near Japan three days in a row, and Japan scrambled fighter jets each time in response.

While China had significantly improved the quality and number of surveillance aircraft operated by its navy and air force over the last decade, Mr Li said he believed coastal air defence radars would be used for routine coverage of the new zone.

Planes - whether surveillance or fighter jets - would be used generally for more specific tasks, he said.

Indeed, attention is likely to focus on airfields and coastal radar stations around Shanghai - strategically placed near the top of the zone.

Independent academic and commercial analysis of China's air force and naval aviation deployments shows a concentration of surveillance aircraft, together with expanding fleets of indigenous J-10 and Russian-acquired Su-30 jet fighters.

An estimated 45 surveillance planes are also within range of the zone, along with as many as 160 fighters around Shanghai - including some ageing locally produced J-7 aircraft.

Most of the surveillance planes are variants of the long-range, locally manufactured Y-8, equipped for separate tasks, such as early-warning patrols, electronic intelligence gathering as well as ship and submarine surveillance.

Particular regional attention is focused on four larger KJ-2000 Airborne Warning and Control System aircraft, converted Russian Il-76 planes, based out of Jiangsu province, neighbouring Shanghai, and within reach of Japan and Taiwan.

"We don't think China's AWACS planes and their abilities are up to the standard of the US and its allies," one Asian military attache in Hong Kong said. "But we can be sure they are getting there - and any extensive enforcement operation could bring them into full play - so we are watching them closely."

The potential behaviour of Chinese pilots during any intensified campaign is also drawing scrutiny - with US officials particularly worried about the risk of miscalculations or accidents.

The days of Chinese fighter pilots buzzing U.S. surveillance planes largely ended when one died in a collision with a US aircraft in 2001.

US military pilots say their Chinese counterparts have generally stopped any fast and loose manoeuvres during routine intercepts after the fatal collision above the South China Sea sparked a crisis in Sino-US ties.

"You just don't see the hot-dogging you used to see up there," one pilot said. "As China's got a lot more assets, its pilots have gotten a lot more professional."

While insisting the zone would be here to stay, Chinese officials and military officers have insisted that Beijing fully intended to comply with international law.

Senior naval advisor, Rear Admiral Yin Zhou, told state broadcaster CCTV that it was illegal to shoot down planes in international airspace.

"Once you enter our territorial airspace we can shoot you down," he said. "But beforehand I would have warned you: if you don't report and enter our territorial airspace, we would take drastic measures."

A Defence Ministry spokesman would not confirm to Reuters whether China's interception aircraft would be armed as they patrolled the zone, however.

"For unidentified or threatening flying objects in the (zone), the Chinese side will, according to different situations, take timely identification, surveillance... and control measures to deal with it," the spokesman said.

"We hope that relevant parties give proactive co-operation to jointly maintain flying safety."