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