SOUTH CHINA SEA (Oct. 3, 2016) The guided-missile destroyers USS Spruance (DDG 111) and USS Decatur (DDG 73) steam alongside amphibious assault ship USS Bonhomme Richard (LHD 6) as part of interoperability drills between the Pacific Surface Action Group (PAG SAG) and Bonhomme Richard Expeditionary Strike Group (ESG). (Photo by PO3 Jeanette Mullinax)

Recent satellite imagery appears to show China is completing structures intended to house surface-to-air missiles (SAMs) on a series of artificial islands in the South China Sea, a Washington think-tank said Thursday.

According to images published by the Asia Maritime Transparency Initiative, the structures are being installed on Fiery Cross Reef, Mischief Reef and Subi Reef in the Spratly Islands.

The AMTI, which is part of the Center for Strategic and International Studies, said China appears to have begun construction on the buildings between late September and early November 2016.

“This indicates they are not reactions to the political cycle in Washington, but rather part of a steady pattern of Chinese militarization,” the group wrote.

China has already installed HQ-9 SAMs on Woody Island, but these are only covered by camouflage netting, AMTI said.

The new structures would provide the SAMs with better protection from seawater and the elements.

Beijing has created seven islets in the Spratly Islands in recent years, built up from smaller land protuberances and reefs.

Although Beijing insists it does not wish to militarize the contested waters of the South China Sea, ongoing satellite imagery has shown the installation of military equipment and longer runways.

In December, AMTI released images showing a series of hexagonal structures in place on each of the seven islets — apparently housing large anti-aircraft guns and close-in weapons systems.