China has built new military facilities on islands in the South China Sea, a U.S. think tank reported on Thursday, a move that could raise tensions with Washington, which has accused Beijing of militarizing the vital waterway.

The Asia Maritime Transparency Initiative (AMTI), part of Washington's Center for Strategic and International Studies, said new satellite images show missile shelters and radar and communications facilities being built on the Fiery Cross, Mischief and Subi Reefs in the Spratly Islands.

The United States has criticized China's build-up of military facilities on the artificial islands and is concerned they could be used to restrict free movement through the South China Sea, an important trade route.