China's land reclamation in the South China Sea is out of step with international rules, US defence secretary Ash Carter told a security conference in Singapore.
He stepped up America's condemnation of the communist giant as Beijing officials sat in the audience, and his remarks were attacked by a Chinese military officer.
Mr Carter said turning underwater land into airfields will not expand China's sovereignty, and he told the room full of Asia-Pacific leaders and that the US opposes "any further militarisation" of the disputed lands.
His remarks were immediately slammed as "groundless and not constructive" by a Chinese military officer in the audience.
Mr 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 try to use the land reclamation projects for military purposes.
The weaponry was discovered at least several weeks ago, and US officials say the vehicles have been removed.
China's behaviour in the South China Sea has become an increasingly sore point in relations with the US, even as President Barack Obama and China's president Xi Jinping have sought more co-operation in other areas, such as climate change.
Pentagon spokesman Brent Colburn said the US was aware of the artillery, but he declined to provide other details.
Defence officials described the weapons as self-propelled artillery vehicles and said they posed no threat to the US or American territories.
While Mr Carter did not refer directly to the weapons in his speech, he told the audience that now is the time for a diplomatic solution to the territorial disputes because "we all know there is no military solution".
He told the audience at the International Institute for Strategic Studies summit: "Turning an underwater rock into an airfield simply does not afford the rights of sovereignty or permit restrictions on international air or maritime transit."
China's actions have been "reasonable and justified," said Senior Colonel Zhao Xiaozhuo, deputy director of the Centre on China-America Defence Relations at the People's Liberation Army's Academy of Military Science.
He challenged Mr Carter, asking whether America's criticism of China and its military reconnaissance activities in the South China Sea "help to resolve the disputes" and maintain peace and stability in the region.
While Mr Carter's criticism was aimed largely at China, he made it clear that other nations who are doing smaller land reclamation projects also must stop.
One of those countries is Vietnam, which he is scheduled to visit during this 11-day trip across Asia. Others are Malaysia, the Philippines and Taiwan.
One senior US defence official has said the US is considering more military flights and patrols closer to the projects in the South China Sea, to emphasise that reclaimed lands are not China's territorial waters.
Officials are also looking at ways to adjust the military exercises in the region to increase US presence if needed.
One possibility would be for US ships to travel within 12 miles of the artificial islands, to further make the point that they are not sovereign Chinese land.
The US has been flying surveillance aircraft in the region, prompting China to file a formal protest.
China has said the islands are its territory and that the buildings and other infrastructure are for public service use and to support fishermen.