Satellite imagery shows that China has put nearly 400 buildings on Subi Reef in the South China Sea.

Data shows that the number of buildings on Subi Reef is about double that on China's other large outposts in the hotly contested region, known as the Spratly Islands.

Experts are concerned about China's increasing militarization of the South China Sea, and they say it may plan to host a large number of troops on Subi Reef.

Satellite imagery shows nearly 400 buildings on a reef occupied by China in the South China Sea, and experts say it indicates Beijing might eventually deploy troops there.

Using images from DigitalGlobe satellites, the nonprofit Earthrise Media analyzed photos of Subi Reef, which is closer to Vietnam and the Philippines than mainland China, and discovered that a large number of buildings, parade grounds, radar equipment, and even basketball courts had been built since 2014.

There were nearly 400 permanent, free-standing buildings, Earthrise's founder, Dan Hammer, told Reuters. Subi has seen the most construction by any country on an island in the South China Sea, the news outlet reported.

Subi is China's largest man-made island within the Spratly archipelago, parts of which are claimed by several countries. Citing Earthrise data, Reuters reported that Subi has about double the number of buildings on each of China's next two largest islands in the region.

The increase in buildings indicates Subi may one day host a large contingent of People's Liberation Army marines, experts say.

Last week, China released footage of H-6K nuclear-capable bombers landing on another island in the South China Sea. Runways and hangars built on Subi could accommodate such bombers.

And on Wednesday, the US uninvited China from a military exercise, citing "China's continued militarization of disputed features in the South China Sea."

Adm. Philip Davidson, the incoming US Pacific Command chief, told a congressional panel last month that "in short, China is now capable of controlling the South China Sea in all scenarios short of war with the United States."

The South China Sea is a highly contentious area with many natural resources that is also one of the world's main shipping corridors. China, Brunei, Taiwan, Malaysia, Vietnam, and the Philippines have competing claims to areas of the sea and its islands.

Data from Earthrise shows that China has more buildings in the South China Sea — 1,652 — than all other claimants put together, Reuters reported.

Davidson said last month that China's growing presence in the South China Sea presented a substantial challenge to regional US military operations, adding that China's military was "executing deliberate and thoughtful force posture initiatives."

"China claims that these reclaimed features ... will not be used for military means, but their words do not match their actions," Davidson said.

He added: "Once occupied, China will be able to extend its influence thousands of miles to the south and project power deep into Oceania. The PLA will be able to use these bases to challenge US presence in the region, and any forces deployed to the islands would easily overwhelm the military forces of any other South China Sea claimants."