WASHINGTON - Newly released satellite images show Vietnam has carried out significant land reclamation at two sites in the disputed South China Sea, though the scale and pace of the work are dwarfed by that of China, a United States research institute said on Thursday.

The photographs, shared with Reuters by the Washington-based Centre for Strategic and International Studies (CSIS), show an expansion of the land area of Vietnamese-controlled West London Reef and Sand Cay in the Spratly archipelago and the addition of buildings.

Dr Mira Rapp-Hooper, director of the CSIS' Asia Maritime Transparency Initiative, said the work include military installations and appear to have started before China began a flurry of reclamation projects last year.

The satellite photographs were taken between 2010 and April 30 this year.

"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," Dr Rapp-Hooper said.

The speed of recent Chinese reclamation work has alarmed its neighbours and the United States, which sees it as a potential threat to the status quo in a region through which US$5 trillion (S$6.7 trillion) of seaborne trade passes each year.

China claims 90 per cent of the South China Sea, which is thought to be rich in oil and gas, with overlapping claims from Brunei, Malaysia, the Philippines, Vietnam and Taiwan.

New Vietnamese military facilities at Sand Cay appear to include defensive positions and gun emplacements, and new buildings visible on West London Reef may also have military applications, Dr Rapp-Hooper said.

Late last month, after weeks of criticism about its reclamation work, China hit back by accusing Vietnam, the Philippines and others of carrying out illegal building work on "Chinese" islands in the South China Sea.

"Strictly speaking, these photos show that China is right," Dr Rapp-Hooper said. "But we can safely say that the scope and scale of what China has undertaken are totally unprecedented and dwarf Vietnam's activities many times over."

She said the images show that Vietnam has reclaimed about 65,000 sq m of land at West London Reef and 21,000 sq m at Sand Cay.

In comparison, China has reclaimed 900,000 sq m at a single reef, Fiery Cross.

Dr Rapp-Hooper said satellite images show that since about March last year, China has conducted reclamation work at seven sites in the Spratlys and is constructing a military-size airstrip on one artificial island and possibly a second on another.

She said Vietnam already has an airstrip in the Spratlys.

The US State Department and Pentagon had no immediate comment on the latest images.

US President Barack Obama last month accused China of "flexing its muscles" to advance its maritime claims, and Washington has been helping countries in the region, including Vietnam, strengthen their defence capabilities.

REUTERS