China had reclaimed 17 times more land in the 20 months as of June in a disputed area of the South China Sea than all other claimants combined over the past 40 years, the U.S. Defense Department said Thursday.

In a first-of-its-kind Asia-Pacific Maritime Security Strategy report, the Pentagon said the work continues but has shifted to construction of infrastructure on the artificially created land in the Spratly Islands, threatening to further destabilize the security situation in the region.

"China has said that it ... has stopped reclamation. ... It's not clear to us that they've stopped," Assistant Defense Secretary David Shear told a Pentagon briefing Friday as the department released the report.