A U.S. think tank says China has built new radar facilities on small island holdings in the South China Sea's disputed Spratly Islands.

The Center for Strategic and International Studies says the radars on the outposts of Gaven, Hughes, Johnson South and Cuarteron reefs could be key to helping China establish effective control over the strategically vital area's sea and airspace.

China has been building islands on top of reefs and atolls it controls, then adding air strips, harbors and other infrastructure that would help consolidate its control. Five other governments also hold maritime claims that overlap with Beijing's, and the U.S. has said it is concerned about China's militarization of the area.

Beijing refutes the accusations, saying it is merely installing defensive measures on islands, primarily for civilian use.