FILE - An aerial view of one of the structures built by China at the Philippine-claimed Mischief Reef, Spratly Islands in South China Sea.
FILE - An aerial view of one of the structures built by China at the Philippine-claimed Mischief Reef, Spratly Islands in South China Sea.

WASHINGTON - China has reclaimed more land in the disputed Spratly Islands of the South China Sea than previous known, according to a new report by the U.S. Defense Department on Beijing's controversial island-building campaign.

The report says that since China began the effort in December 2013, it has reclaimed land at seven of its eight Spratly outposts.  The document (Asia-Pacific Maritime Security Strategy) notes this is 17 times more land in nearly two years than other claimants combined in the past 40 years.

South China Sea Territorial Claims
South China Sea Territorial Claims

China has been creating the artificial islands in waters where countries that include Vietnam, Malaysia, Taiwan and the Philippines have overlapping claims.

China says it stopped the reclamation work in the South China Sea in June.  

"It's not clear to us that they have stopped or if they are finishing up," said David Shear, the assistant secretary of defense for Asian and Pacific security affairs.

He told reporters Friday at the Pentagon that the U.S. will continue to watch that situation very closely and engage with the Chinese as the U.S. has in the recent past on this subject.  The reclamation, he said, is taking place on "features"  that have long been occupied by China.

United States Navy lieutenant commander Kirk Benso
United States Navy Lieutenant Commander Kirk Benson, points at a map of the South China sea at the Tactical Flag Command Center (TFCC) of the 19,200-ton USS Blue Ridge in Manila, July 22, 2014.

The Pentagon's report notes that Beijing is completing work on an air strip on one of its man-made outposts.

It says once the airstrip on Fiery Cross Reef is operational, China could potentially use it as an alternative runway for carrier-based planes, allowing its military to conduct "sustained operations" with aircraft carriers in the area.

Since the beginning of 2015, satellite images of the reclamation work have shown shoals and reefs being turned into artificial islands.  Beijing has said it has "indisputable sovereignty" over practically the entire sea.

The Pentagon report says the U.S. is working to maintain the necessary capabilities to deter conflict and reassure allies and partners.

U.S. Defense Secretary Ash Carter said Thursday the United States will continue to fly, sail and operate wherever international law permits. 

"As we've always had the right to do, we will continue to do that, and none of this is going to change our conduct in any way," he said.

The Pentagon report comes ahead of a state visit to the U.S. next month by Chinese President Xi Jinping.