Newly released satellite imagery shows that Vietnam has been engaged in significant land reclamation projects at two sites in the disputed waters of the South China Sea, a U.S. research group said Thursday. The photographs, which were obtained by Reuters from the Centre for Strategic and International Studies (CSIS), show land expansions in the Vietnam-controlled Sand Cay and West London Reef, both of which are part of the Spratly Islands.

Mira Rapp-Hooper of CSIS told Reuters that the work appeared to include military installations, and likely started before China initiated its own massive construction and reclamation projects last year. 

New buildings have been observed in the area as well. "On one site, it has constructed a significant new area that was formerly under water and at another it has used land reclamation to add acreage to an existing island," Rapp-Hooper reportedly said.

The new Vietnamese facilities appear to include defensive structures and gun emplacements, she said. China lays territorial claim to 90 percent of the South China Sea under its so-called “nine-dash line,” which has led to frequent disputes with other countries.

Brunei, Malaysia, the Philippines, Vietnam and Taiwan have conflicting claims with China, and have accused it of rapidly and significantly expanding its projects in the region -- including major land reclamation and construction of military facilities -- in violation of agreed norms. Recent satellite imagery has reportedly shown that China is building a runway on reclaimed territory, which could be used to host military aircraft. China has defended its construction, saying it intends to expand its search and rescue capabilities in the region.

Rapp-Hooper reportedly said that the scope of the newly revealed Vietnamese expansion is dwarfed by China’s so-called “great wall of sand,” which has resulted in new land mass spreading over 1.5 square miles. In comparison, Vietnam’s project spans about 0.03 square miles across West London Reef and Sand Cay. "Strictly speaking, these photos show that China is right, but we can safely say that the scope and scale of what China has undertaken is totally unprecedented and dwarfs Vietnam’s activities many times over," she told Reuters.

The U.S. has accused China of “flexing its muscles” to advance its regional claims, and Washington has reaffirmed its support to allies by hosting massive joint military exercises in cooperation with the Philippines in recent weeks.

China has accused several countries in the region, including the Philippines and Vietnam, of engaging in their own construction projects, which it says violates an informal 2002 accord between members of the Association of South East Nations (ASEAN) regional bloc. ASEAN’s chief has reportedly previously said the group “can’t accept” China’s sweeping territorial claims.