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 seven artificial islands China has created in the Spratly archipelago.
The images show a retaining wall around an area 3,000 meters (3,280 yards) long, matching similar work by China on two other reefs in the Spratlys, Subi and Fiery Cross, said Greg Poling, director of CSISs Asia Maritime Transparency Initiative (AMTI).
Poling said the work "more likely than not indicates preparations for a runway" on the reef.
Satellite photographs from late June showed China had almost finished a 3,000-meter airstrip on Fiery Cross.
Poling said other satellite photos from last week showed work was advancing at Subi Reef, where "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."
Asked about Mischief Reef on Monday, China's Foreign Ministry spokesman Hong Lei repeated Chinas claim to "indisputable sovereignty" over the Spratly islands and its right to establish military facilities there.
Security experts say 3,000-meter airstrips 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 advancing 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 US Defense Department, Commander Bill Urban, declined to comment specifically on Polings assessment, but repeated US calls for a halt to land reclamation, construction and militarization of South China Sea outposts to "ease tensions and create space for diplomatic solutions."
"Chinas 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 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.
The Philippine government had no immediate comment. China stepped up creation of artificial islands in the South China Sea last year, drawing strong criticism from Washington.