Satellite photographs taken in late July show China appears to have built reinforced aircraft hangars on its holdings in disputed South China Sea islands, a Washington-based research group says.

The hangars on Fiery Cross, Subi and Mischief Reefs in the Spratly islands have room for any fighter jet in the Chinese Air Force, the Centre for Strategic and International Studies (CSIS) said in a report on the photographs.

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

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

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

CSIS said that apart from a brief visit to Fiery Cross Reef by a military transport plane earlier in the year, "there is no evidence that Beijing has deployed military aircraft to these outposts".

The rapid construction of the hangars, however, "indicates that this is likely to change".

A US defence official said it was unlikely the hangers would be used for civilian purposes.

The official added, however, that the Chinese move was seen as skirting around the line rather than crossing it, and there would be increased concern if China actually moved in military aircraft and started using a reef as a forward operating base. China has repeatedly denied doing so and has in turn criticised US 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. "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."

The hangars all show signs of structural strengthening, CSIS said. Other facilities including unidentified towers and hexagonal structures have also been built on the islets in recent months, CSIS said.

- Reuters, AAP