The rapid construction of artificial islands in the strategic waters comes to 2,000 acres, with 75 per cent of the total in the last five months, officials said.
"China has expanded the acreage on the outposts it occupies by some four hundred times," said a US defense official.
The United States did not endorse land reclamation by any of the countries with territorial claims in the South China Sea, but "the pace and scale of China's land reclamation in recent years dwarfs that of any other claimant," the official said.
The commander of the US Pacific Fleet, Admiral Harry Harris, said in March that China is "creating a Great wall of sand."
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US officials released the reclamation estimate as the Pentagon issued its annual report to Congress on the state of China's military, which repeated accusations that Beijing was staging cyber attacks to scoop up information on American defense programs.
Previous reports have noted China's focus on cyber and space weapons but this year's document included a special section on the country's massive dredging and island building in the strategic South China Sea.
At four reclamation sites, China has moved from dredging operations to "infrastructure development" that could include harbors, communications and surveillance systems, logistics support and "at least one airfield," the report said.