The new satellite images show China has built infrastructure covering 72 acres (28 hectares) of the Spratly and Paracel islands this year.
The Washington-based Asia Maritime Transparency Initiative (AMTI) of the Center for Strategic and International Studies reported on Thursday that hangars, underground storage, missile shelters, radar arrays and other facilities have been built.
The building follows land reclamation by China which was completed in 2016 in the Spratlys. The island chain is also subject to territorial claims from Vietnam, Malaysia, Taiwan, the Philippines and Brunei.
The US Pentagon says China has added more than 3,200 acres of land to the seven territories it occupies in the area.
A question of use
China says the artificial islands in the Spratlys with their airstrips and military installations are mainly for civilian purposes and to safeguard fishing and maritime trade.
The AMTI reported that China has deployed new military aircraft, including J-11B fighter planes used in drills. In November there were Y-8 transport aircraft believed to be capable of electronic intelligence gathering.
The US has accused China of militarizing the region and changing its geography to strengthen its claims across the South China Sea. Washington believes China will use the islands and its military presence on them to deny access to strategic routes.
In the Paracel islands, a new helipad, wind turbines and large radar towers had been built. The radar towers on Triton Island could be used in China's disputes with Vietnam and with the US over freedom of navigation operations, which the US navy has used to back its perceived right to free passage in international waters.
Concern from China's neighbors
China's engineering feats in the region have raised concerns from its neighbors, especially the Philippines which brought a case to the Permanent Court of Arbitration in The Hague which ruled in July 2016 against China. But Beijing refused to acknowledge either the decision or the court's jurisdiction.
After the election of President Rodrigo Duterte in the Philippines, the new government adopted a more conciliatory attitude towards China.
The US has also moved its focus away from China's activities in the South China Sea to North Korea and its nuclear programs. Greg Poling, the AMTI director said: "It's gotten off the front pages, but we shouldn't confuse that with a softening in China's pursuit of its goals. They are continuing all the construction they want."
jm/se (AP, Reuters)