WASHINGTON - China appears to be building a third airstrip in contested territory in the South China Sea, a US expert said on Monday, citing satellite photographs taken last week.The photographs taken for Washington's Center for Strategic and International Studies (CSIS) think tank on Sept. 8 show construction on Mischief Reef, one of several artificial islands China has created in the Spratly archipelago.The images show a rectangular area with a retaining wall, 3,000 meters (3,280 yards) long, matching similar work by China on two other reefs, Subi and Fiery Cross, said Greg Poling, director of CSIS's Asia Maritime Transparency Initiative (AMTI)."Clearly, what we have seen is going to be a 3,000-meter airstrip and we have seen some more work on what is clearly going to be some port facilities for ships," he said.Security experts say the strip would be long enough to accommodate most Chinese military aircraft, giving Beijing greater reach into the heart of maritime Southeast Asia, where it has competing claims with several countries.News of the work comes ahead of a visit to Washington next week by Chinese President Xi Jinping. US worries about China's increasingly assertive territorial claims are expected to be high on the agenda.A new airstrip at Mischief Reef would be particularly worrying for the Philippines, a rival claimant in the South China Sea. It would allow China to mount "more or less constant" patrols over Reed Bank, where the Philippines has long explored for oil and gas, Poling said.Three airstrips, once completed, would allow China to threaten all air traffic over the features it has reclaimed in the South China Sea, he said, adding that it would be especially worrying if China were to install advanced air defenses.Satellite photographs from late June showed China had almost finished a 3,000-meter airstrip on Fiery Cross.Satellite images from earlier this year showed reclamation work on Subi Reef creating land that could accommodate another airstrip. Poling said the latest images made it obvious that such an airstrip was being built at Subi.China stepped up creation of artificial islands in the South China Sea last year, drawing strong criticism from Washington, which has spoken against militarization of outposts there.Asked about Mischief Reef on Monday, China's Foreign Ministry spokesman Hong Lei repeated China's claim to "indisputable sovereignty" over the Spratly Islands and its right to establish military facilities there.