HONG KONG: China's apparent construction of a third airstrip on its man-made islands in the disputed South China Sea could fill a gap in Beijing's anti-submarine defenses, complicating operations for the U.S. Navy and its allies, Chinese and Western experts said.

While most attention has been on the power projection China would get from its new islands in the Spratly archipelago, China could also use them to hunt rival submarines in and beyond the strategic waterway, they said. Possessing three airstrips more than 1,400 km (870 miles) from the Chinese mainland would enable Beijing to extend the reach of Y-9 surveillance planes and Ka-28 helicopters that are being re-equipped to track submarines, the experts added. A Pentagon report in May noted China lacked a robust anti-submarine warfare capability off its coastline and in deep water.

Strengthened anti-submarine capabilities could also help China protect the movements of its Jin-class submarines, capable of carrying nuclear-armed ballistic missiles and which are at the core of China's nuclear deterrence strategy, said Zhang Baohui, a mainland security specialist at Hong Kong's Lingnan University. "That would provide greater security for China's nuclear submarines to survive ... and if necessary to execute their orders in wartime," Zhang told Reuters. "They would be safer than in open oceans where China cannot provide adequate support."

The artificial islands, built on seven reefs over the last two years, will be high on the agenda when Chinese President Xi Jinping has talks with President Barack Obama in Washington next week. Washington has criticized the reclamation and construction. China, increasingly confident about its military firepower, has repeatedly stressed it has "indisputable sovereignty" over the entire Spratlys, saying the islands would be used for civilian and undefined military purposes.

Foreign Minister Wang Yi on Wednesday said "necessary" construction work would improve conditions on the islands. Satellite photographs show construction is almost finished on a 3,000-metre-long (10,000-foot) airstrip on Fiery Cross Reef. Recent images showed Subi Reef would also have a 3,000-metre airstrip, Greg Poling, director of the Asia Maritime Transparency Initiative at the Center for Strategic and International Studies think tank in Washington, said on Monday.

Poling, citing images taken last week, said China also appeared to be doing preparatory work for an airstrip on Mischief Reef. Together, the three islands form a rough triangle in the heart of the Spratlys, where the Philippines, Vietnam, Malaysia, Brunei and Taiwan all have competing claims. While a noisy and relatively shallow operating environment for submarines, the South China Sea has several deep water channels giving access to the Indian and Pacific oceans. Asked if Washington was concerned the airstrips would enhance China's anti-submarine capabilities, a Pentagon spokesman, Commander Bill Urban, said the United States was monitoring events in the South China Sea.

In a speech on Wednesday, U.S. Defense Secretary Ash Carter said the United States would "fly, sail, and operate wherever international law allows". "Turning an underwater rock into an airfield simply does not afford the rights of sovereignty or permit restrictions on international air or maritime transit," Carter told a U.S. Air Force conference. One mainland-based naval analyst said China was trying to improve sonar and other detection equipment carried aboard its Y-9 patrol planes and Ka-28 helicopters. China was also expected to put detection devices on the seabed around the new islands, creating "an electronic gateway", he added.

Zhang has previously said ballistic missile submarines are more important for China's nuclear deterrent than other powers given Beijing's policy, dating back to the 1960s, of only using nuclear weapons if attacked with them first. This means China's land-based weapons would be vulnerable to a first strike if Beijing stuck to its "no first use" policy in a conflict. Chinese media and international military blogs this year have shown photographs of Jin-class submarines operating from a naval base on Hainan Island off southern China. It's unclear if they have been armed with long-range JL-2 nuclear ballistic missiles.

The Pentagon report said four Jin-class submarines were operational, with a fifth expected to be added. "China will likely conduct its first (submarine) nuclear deterrence patrol sometime in 2015," the report said. The importance of that deterrence means China is likely to eventually impose an Air Defence Identification Zone (ADIZ) over part of the South China Sea, security experts say, mirroring its declaration of such a zone over the East China Sea in late 2013. In a return to Cold War-style cat-and-mouse operations undersea, rival submarines were already trying to track each other, said Western and Asian naval officers with experience of anti-submarine warfare.

They said the United States would be trying to identify and track individual Chinese submarines, just as it stalked then-Soviet Union missile submarines across the Pacific and Atlantic oceans during the Cold War. Japan's ultra-quiet diesel-electric submarines were also increasingly active while, over time, Vietnam's emerging fleet of advanced Russian-built Kilo-class submarines would be another headache for China. "We're looking at them, and now increasingly they are looking at us," one retired Asian-based naval officer said of China's growing undersea operations.