US calls for land reclamation 'halt' in South China Sea
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".
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".
"China has reclaimed over 2,000 acres, more than all other claimants combined and more than in the entire history of the region. And China did so in only the last 18 months," he said.
"It is unclear how much farther China will go. That is why this stretch of water has become the source of tension in the region and front-page news around the world."
The US defence secretary also said the US would maintain a substantial presence in the region, adding: "The United States will fly, sail, and operate wherever international law allows."
Analysis: BBC defence correspondent Jonathan Marcus
Ash Carter's comments were some of the toughest on China's island-building strategy that have come from a senior US official.
The key question is what the US can actually do about it beyond words.
Recent over-flights by US maritime patrol aircraft of some of the new islands have been met by terse radio traffic with the Chinese demanding they leave the area. The fear is that this kind of activity might lead to some kind of incident in the air or at sea that may only further inflame tensions between Washington and Beijing.
China takes the view that it is doing nothing wrong - and certainly nothing that other countries are not also doing.
However, it is clearly the pace and scale of what China is doing that worries many.
And with the precedent of China's self-declared air defence identification zone in the East China Sea, there are fears that such an approach to monitoring or potentially even ultimately limiting freedom of movement could be extended into the South China Sea as well.
Senior Colonel Zhao Xiaozhuo, a member of China's delegation at the conference, said that China's actions were "reasonable and justified", and that "it is wrong to criticise China for affecting peace and stability through construction activities".
Mr Carter's comments follow reports from US defence officials that China had put two artillery vehicles on one of the artificial sands it is creating in the disputed Spratley Islands area of the South China Sea earlier this month.
Asked about the US claims, China's foreign ministry spokeswoman Hua Chunying said she was "not aware of the situation" described, and urged the US to "stop making any provocative remarks".
China has previously said its work in the area is legal and needed to safeguard its sovereignty.