An aerial photo taken though a glass window of a Philippine military plane shows the alleged on-going land reclamation by China on mischief reef in the Spratly Islands in the South China Sea, west of Palawan, Philippines, May 11, 2015. REUTERS/Ritchie B. Tongo/Pool

By Greg Torode

HONG KONG (Reuters) - When the U.S. navy sent a littoral combat ship on its first patrol of the disputed Spratly islands in the South China Sea during the past week, it was watching the skies as well.

The USS Fort Worth, one of the most modern ships in the U.S. navy, dispatched a reconnaissance drone and a Seahawk helicopter to patrol the airspace, according to a little-noticed statement on the navy's website.

While the navy didn't mention China's rapid land reclamation in the Spratlys, the ship's actions were a demonstration of U.S. capabilities in the event Beijing declares an Air Defence Identification Zone (ADIZ) in the area - a move experts and some U.S. military officials see as increasingly likely.

"It's not inevitable but if we are betting paychecks I'll bet that they will eventually declare one, I just don't know when," said a senior U.S. commander familiar with the situation in Asia.

ADIZs are not governed by formal treaties or laws but are used by some nations to extend control beyond national borders, requiring civilian and military aircraft to identify themselves or face possible military interception.

China sparked condemnation from the United States and Japan when it imposed an ADIZ in the East China Sea, above uninhabited islands disputed with Tokyo, in late 2013.

Chinese military facilities now under construction on Fiery Cross Reef in the Spratlys, including a 3,000-metre (10,000-foot) runway and airborne early warning radars, could be operational by the year-end, said the U.S. commander, who declined to be identified.

Recent satellite images also show reclamation work on Subi Reef creating landmasses that, if joined together, could make space for a similar sized airstrip.

Growing concern in Washington that China might impose air and sea restrictions in the Spratlys once it completes work on its seven artificial islands is likely to be on the agenda when U.S. Secretary of State John Kerry meets Chinese leaders in Beijing this weekend for previously scheduled talks.

TOUGH TO ENFORCE

Asia's rising power claims sovereignty over most of the South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei also have overlapping claims.

China has said it had every right to set up an ADIZ but that current conditions in the South China Sea did not warrant one.

Enforcing such an ADIZ would be difficult even with two airstrips capable of handling fighter planes in the Spratlys, as well as an expanded airstrip on Woody island in the disputed Paracel island chain further north because of the distances involved, regional military officials and experts said.

The Spratlys for example lie more than 1,100 km (680 miles) from the Chinese mainland, putting China's well-equipped airbases along its coastline well out of reach.

"Even with the new reclamations, it is going to be a stretch for China to routinely enforce such a zone that far south," said Richard Bitzinger, a regional security analyst at Singapore's S. Rajaratnam School of International Studies.

The Japanese and U.S. military ignore the ADIZ above the East China Sea, as does Japan's two major carriers, ANA Holdings <9202.T> and Japan Airlines <9201.T>.

A study produced by the independent U.S. Congressional Research Service earlier this year noted that while China's air force actively monitors that zone with ground radar from its coastline, it had generally shown restraint in enforcement.

China's planes were unlikely to maintain a constant presence over the East China Sea, the study noted, citing a U.S. air force assessment.

RISK OF ESCALATION

The South China Sea might prove more problematic for China given the complexity of the dispute and the possibility of challenges from the U.S. navy and air force.

Indeed, on Tuesday, a U.S. official said the Pentagon was considering sending military aircraft and ships to assert freedom of navigation around the Chinese-made islands.

China's Foreign Ministry responded by saying Beijing was "extremely concerned" and demanded clarification.

On Friday it accused the Philippines of working together with the United States to "exaggerate the China threat" over the Spratlys.

China had recently warned Philippine air force and navy planes at least six times to leave the Spratlys, the Philippine military commander responsible for the region said last week. The planes refused.

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