China was constructing 24 fighter-sized hangars, fixed-weapons positions and other military-related infrastructure on each of three major Chinese-occupied features in the disputed South China Sea as of late last year, the U.S. Defense Department said Tuesday.

"China's Spratly Islands outpost expansion effort is currently focused on building out the land-based capabilities of its three largest outposts — Fiery Cross, Subi and Mischief Reefs — after completion of its four smaller outposts early in 2016," the Pentagon said in an annual report to Congress on military and security developments involving China.

"Once all these facilities are complete, China will have the capacity to house up to three regiments of fighters in the Spratly Islands," whose ownership is disputed between Beijing and its smaller neighbors such as the Philippines and Vietnam, the report said.