"We currently assess that China has reclaimed approximately 2,000 acres (800 hectares) of land in the South China Sea," a Department of Defence (DoD) official said.
"We do not believe that large-scale land reclamation is consistent with the region's desire for peace and stability," the official said.
The new estimate, disclosed for the first time, came as the Pentagon released its annual report of military and security developments involving China.
It warned that while the intent of China's construction in the contested islands is unclear, the five emerging outposts could be used for surveillance systems, harbours, an airfield and logistical support.
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The report said at four reclamation sites, China moved from land reclamation operations to infrastructure development and delivered scores of heavy construction equipment to all five reclamation sites.
China's efforts in the strategically vital shipping lane concerns the US, which feels Beijing's move carry a military dimension that could potentially undermine its influence in the Pacific. The Sea is believed to be rich in oil and gas.
As of December 2014, China had reclaimed about 500 acres of land as part of this effort.
"At reclamation sites in the infrastructure phase of development, China excavated deep channels and built new berthing areas to allow access for larger ships to the outposts," the Pentagon report said.
It said the ultimate purpose of the expansion projects remains unclear.
China has stated these projects are mainly for improving the living and working conditions of those stationed on the islands.
"No Chinese-occupied outpost in the Spratly Islands has an airfield or secure docking, unlike other claimant nations."
According to Pentagon, Taiwan began a modest land reclamation effort at Itu Aba Island by April 2014 and to date has reclaimed nearly five acres near the island's airstrip.
According to reports, Taiwan is building a USD 100-million port next to the airstrip that is designed to accommodate 3,000-ton naval frigates and coast guard cutters, it said.