China has reclaimed more than 2,900 acres of land from the South China Sea in less than two years in an intensive island-building campaign in the disputed Spratly Islands, a new Pentagon report says.

"By undertaking these actions, China is unilaterally altering the physical status quo in the region, thereby complicating diplomatic initiatives that could lower tensions," the report warned.

With its rich fisheries and oil and gas potential, the South China Sea has been a source of contention for decades.

China, Vietnam and Taiwan claim all 200 or so of the Spratlys land features, while Brunei, the Philippines and Malaysia claim parts of them.

Although not the first or only country to dredge up sand to expand reefs and rocks into artificial islands in the region, the scope and scale of China's land reclamation activities dwarf those of its rival claimants, the report on Asia-Pacific Maritime Strategy said.

"China has now reclaimed 17 times more land in 20 months than the other claimants combined over the past 40 years, accounting for approximately 95 percent of all reclaimed land in the Spratly Islands," the report said.

It has reclaimed land on seven of its eight outposts in the Spratlys, "and as of June 2015, had reclaimed more than 2,900 acres of land."

On all its reclamation sites, China has either started building infrastructure or staged equipment to develop it.

While it was still unclear what China intends to build on the reclaimed land, the report noted that Beijing has said the outposts will have a military component as well as civilian functions.

At some sites, it has dug deep channels and built berthing sites for larger ships.

It is building an air strip on Fiery Cross Reef that is more than twice the length of air strips built on rival outposts by Vietnam, the Philippines, Taiwan and Malaysia.

In the Pentagon's view, the activity appears to be part of an overall maritime strategy to increase China's control in both the East and South China Seas without escalating to a military conflict.

China has been rapidly modernizing its naval forces and coast guard fleet to enforce its claims, and the artificial islands "would enable it to establish a more robust power projection presence into the South China Sea," the report said.

"Its latest land reclamation and construction will also allow it to berth deeper draft ships at outposts; expand its law enforcement and naval presence farther south into the South China Sea; and potentially operate aircraft - possibly as a divert airstrip for carrier-based aircraft - that could enable China to conduct sustained operations with aircraft carriers in the area," the report asserted.