Recent satellite photographs show China appears to have built reinforced aircraft hangars on its holdings in the disputed South China Sea, according to a Washington-based think tank.

Pictures taken in late July show the hangars constructed on Fiery Cross, Subi and Mischief Reefs in the Spratly islands, have room for any fighter jet in the Chinese air force, the Center for Strategic and International Studies (CSIS).

"Except for a brief visit by a military transport plane to Fiery Cross Reef earlier this year, there is no evidence that Beijing has deployed military aircraft to these outposts. But the rapid construction of reinforced hangars at all three features indicates that this is likely to change," CSIS said in a report.

China claims most of the South China Sea, through which $5 trillion in ship-borne trade passes every year. The Philippines, Vietnam, Malaysia, Taiwan and Brunei have overlapping claims.

The images have emerged about a month after an international court in The Hague ruled against China's sweeping claims in the resource-rich region, a ruling emphatically rejected by Beijing.

The United States has urged China and other claimants not to militarize their holdings in the South China Sea.

China has repeatedly denied doing so and has in turn criticized U.S. patrols and exercises for ramping up tensions.

“China has indisputable sovereignty over the Spratly islands and nearby waters," China's Defence Ministry said in a faxed response to a request for comment on Tuesday.

"China has said many times, construction on the Spratly islands and reefs is multipurpose, mixed, and with the exception of necessary military defensive requirements, are more for serving all forms of civil needs."

Ties around the region have been strained in the lead-up to and since The Hague ruling.

China has sent bombers and fighter jets on combat patrols near the contested South China Sea islands, state media reported on Saturday, and Japan has complained about what it has said were multiple intrusions into its territorial waters around another group of islands in the East China Sea.

The hangars all show signs of structural strengthening, CSIS said.

"They are far thicker than you would build for any civilian purpose," Gregory Poling, director of CSIS's Asia Maritime Transparency Initiative, told the New York Times, which first reported on the new images. "They're reinforced to take a strike."

Other facilities including unidentified towers and hexagonal structures have also been built on the islets in recent months, CSIS said.