The photographs taken for Washington's Center for Strategic and International Studies (CSIS) think tank on Sept. 8 show construction on Da Vanh Khan (Mischief Reef) - one of several artificial islands China has created in Vietnam’s Truong Sa (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, Da Xu Bi (Subi Reef) and Da Chu Thap (Fiery Cross Reef), 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. U.S. worries about China's increasingly assertive territorial claims are expected to be high on the agenda.
A spokesman for the U.S. Defense Department, Commander Bill Urban, declined to comment specifically on Poling's assessment, but repeated U.S. calls for a halt to land reclamation, construction and militarization of East Vietnam Sea outposts to "ease tensions and create space for diplomatic solutions."
"China's stated intentions with its program, and continued construction, will not reduce tensions or lead to a meaningful diplomatic solution," he added.
A new airstrip at Da Vanh Khan would be particularly worrying for the Philippines, a rival claimant in the East Vietnam 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 East Vietnam 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 Da Chu Thap.
Satellite images from earlier this year showed reclamation work on Da Xu Bi creating land that could accommodate another airstrip. Poling said the latest images made it obvious that such an airstrip was being built at Da Xu Bi.
China stepped up creation of artificial islands in the East Vietnam Sea last year, drawing strong criticism from Washington.