The US has condemned China's land reclamation in the South China Sea, saying it is out of step with international rules.
Defence secretary Ash Carter called for an "immediate and lasting halt" of the works during the annual Shangri-La Dialogue, an international security conference, in Singapore on Saturday.
"First, we want a peaceful resolution of all disputes. To that end, there should be an immediate and lasting halt to land reclamation by all claimants," Carter said at the conference.
A Chinese military officer, also at the conference, immediately disputed the comments as "groundless and not constructive".
Carter's comments came as defence officials revealed that China had put two large artillery vehicles on one of the artificial islands it is creating in the South China Sea.
The discovery, made at least several weeks ago, fuels fears in the US and across the Asia-Pacific that China will use the land reclamation projects for military purposes.
Last week the Chinese military ordered a US Navy P-8 Poseidon surveillance aircraft to leave an area above the heavily disputed Spratly Islands. But the US plane ignored the demand.
"There should be no mistake: the United States will fly, sail, and operate wherever international law allows, as US forces do all around the world," Carter said in Singapore.
He acknowledged that other claimants have developed outposts of differing scope and degree, including Vietnam with 48, the Philippines with eight, Malaysia with five and Taiwan with one.
"Yet, one country has gone much farther and much faster than any other, and that's China."
"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."
Al Jazeera's Marga Ortigas, who's been following events at the conference explained "freedom of navigation has basically been assured over 70 years, greatly because of the US presence. Should there be a tip in that power balance, there's fear the chaos will result in instability in the region".
Ortigas added that nothing "concrete in terms of policy or agreements" is likely to come from the conference, but that it did allow for a "comfortable, stable space for constructive discussions."
China claims nearly all of the South China Sea, on the basis of lines on Chinese maps published in the 1940s and locking it into disputes with several Southeast Asian neighbours - including Vietnam, the Philippines and Malaysia.
Source: Al Jazeera and agencies