The US has called for an "immediate and lasting halt" to land reclamation in disputed areas of the South China Sea.
US Defence Secretary Ashton Carter told the Shangri-La Dialogue in Singapore that China's actions in the area were "out of step" with international rules.
China claims almost the whole of the South China Sea, resulting in overlapping claims with its neighbours.
Chinese officials have described US remarks on the South China Sea as "groundless and not constructive".
'Peaceful resolution'
Other countries have accused China of illegally taking land to create artificial islands with facilities that could potentially be for military use.
At the conference on Saturday, which was attended by defence ministers from across the Asia-Pacific region, Mr Carter said he wanted the "peaceful resolution of all disputes".
"To that end, there should be an immediate and lasting halt to land reclamation by all claimants," he said.
He acknowledged that other claimants such as Vietnam, the Philippines, Malaysia and Taiwan had reclaimed pockets of land or built outposts in the area, but said "one country has gone much farther and much faster than any other".