The photos – taken for Washington's Center for Strategic and International Studies (CSIS) – show construction on Mischief Reef, one of the artificial islands China has built in the Spratly archipelago, Reuters reported.
The images show infrastructure matching similar work China has done on two other reefs, 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 in the US say the strip would be long enough to accommodate most Chinese military aircraft, giving Beijing greater reach into the South China Sea, where it has competing claims with several countries, Reuters reported.
With three airstrips, China could threaten all air traffic over all its reclaimed territory in the South China Sea, Poling said. He added that it would be especially worrying if China were to install advanced air defenses.
The United States has called China to put a stop to its land reclamation projects in the South China Sea, through which $5 trillion in trade passes every year.
The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims in the region.
China claims "indisputable sovereignty" over the Spratly Islands and its right to establish military facilities there. Beijing has repeatedly stated the artificial islands will be used primarily for humanitarian purposes.