The images, taken September 8, come after China pledged to bring land reclamation in those waters to a halt, and will make for uncomfortable discussions when Chinese President Xi Jinping visits Washington next week.
Greg Poling, the director of CSIS's Asia Maritime Transparency Initiative
, said that China had reclaimed a flat rectangle of land with a retaining wall about 3,000 meters (3,280 yards) in length on Mischief Reef. It's similar to airstrips that China has been building on artificial islands at the Fiery Cross and Subi reefs in the contested Spratly Islands.
"If it does turn out to be a runway, China will have three airstrips that can carry any plane the PLA (People's Liberation Army) has to offer," he said.
"All of these places have gone from being outposts on stilts to full islands, potentially with airstrips, within the space of a year or just over a year," he added.
Messy territorial claims
The South China Sea is the subject of numerous rival -- often messy -- territorial claims, with China, Brunei, Malaysia, the Philippines, Taiwan and Vietnam disputing sovereignty of several island chains and nearby waters.
In June, China had said that island building in the South China Sea was "almost complete
," although it did say that it would continue to build facilities on the islands it has already created.
Poling said that one Chinese dredger was active a week ago, widening a shipping channel.
China has repeatedly said its activity in the South China Sea does not target any other country or affect freedom of navigation by sea or air.
Vietnam, Philippines, Taiwan and Malaysia also have airstrips in the disputed waters but, unlike China's, none could handle a fourth generation fighter jet, Poling said.
None of the three airstrips China has been building are operational but Poling said that China had been laying paint on the runway at the most advanced -- Fiery Cross.
Last year, China stepped up land reclamation on several reefs close to the Spratly Islands (called Nansha by China), alarming its Asian neighbors.
The United States says that China has reclaimed some 2,000 acres -- or the equivalent of 1,500 football fields -- in the past 18 months and has called for an "immediate and lasting" halt to the island building.
Top tier issue
Poling said that China's land reclamation would be a "top tier" issue when President Obama and President Xi meet next week.
"It's going to color the visit. There were already going to be uncomfortable discussions and this isn't going to make it easier," he said.
Chinese navy ships entered U.S. territorial waters off Alaska earlier this month, coming within 12 miles of the coastline during President Obama's visit
to the state, U.S. officials told CNN.
The officials emphasized that China's actions were consistent with "innocent passage" under international maritime law.
However, in the South China Sea, the United States has never breached the 12-mile limit accorded under maritime law -- even though the U.S. doesn't recognize the islands as Chinese territory, and their man-made nature may disqualify them from having any maritime zone.
"Some people think it would be a good idea to (cross the limit) as it sends a message and will put the onus on China to explain what it's doing there," Poling said.