Adm. Scott Swift also criticised China-Russia joint naval exercises planned next month in the South China Sea, saying the choice of location was not conducive to "increasing the stability within the region."
He also said any decision by China to declare an air defense identification zone over the strategic water body would be "very destabilizing from a military perspective."
Attention has been fixed on the South China Sea since the July 12 ruling by The Hague-based Permanent Court of Arbitration in a case brought by the Philippines.
China refused to participate in the case or recognize the ruling, and strongly criticised the US for encouraging its ally to pursue the matter.
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Since then, Beijing has launched air patrols over the South China Sea, said it would consider declaring an air defense zone and vowed to continue work on man-made islands created from piling sand atop coral reefs in the highly contested Spratly group.
The photos were collected and studied by the Center for Strategic and International Studies, a Washington-based think tank, and reported in The New York Times.
They show construction work on man-made islands at Fiery Cross, Subi and Mischief reefs. China has said the new islands are primarily to assist fishermen and other causes, as well as to reinforce its sovereignty claims.
China also says that the islands should be able to defend themselves, and that it is entitled to build whatever structures it wishes on them.