The Chinese government has nearly completed a 3km-long airstrip on a man-made island built to project military power into the South China Sea.
New photos taken by US aircraft show dozens of Chinesedredging vessels at work near the island in the Spratly archipelago.
The Asia Maritime Transparency Initiative at the Centre for Strategic and International Studies in Washington says that two helipads, ten satellite dishes, a Chinese Naval vessel and a radar tower are visible in the photo.
China claims most of the South China Sea as its own territory, though this is disputed by neighbouring nations including the Philippines, Vietnam, Malaysia, Brunei and Taiwan.
Last week the US State Department compared China’s policy in the South China Sea to that of Vladimir Putin’s Russia in eastern Ukraine.
“In both eastern Ukraine and the South China Sea, we’re witnessing efforts to unilaterally and coercively change the status quo — transgressions that the United States and our allies stand united against,”
Mr Blinken was speaking at a speech at the Center for a New American Security think tank.
China intensified efforts to construct the island last year.
The country’s government says the outposts have military, disaster relief, navigation, and search and rescue purposes.