China’s land reclamation in the South China Sea is out of step with international rules, and turning underwater land into airfields won’t expand its sovereignty, Defense Secretary Ash Carter told an international security conference Saturday, stepping up America’s condemnation of the communist giant as Beijing officials sat in the audience.

Carter told the room full of Asia-Pacific leaders and experts that the U.S. opposes “any further militarization” of the disputed lands.

His remarks were immediately slammed as “groundless and not constructive” by a Chinese military officer in the audience.

Carter’s comments came as defense 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 U.S 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 two U.S. officials who are familiar with intelligence about the vehicles say they have been removed. The officials weren’t authorized to discuss the intelligence and spoke only on condition of anonymity.

The Pentagon would not release any photos to support its contention that the vehicles were there.

China’s assertive behavior in the South China Sea has become an increasingly sore point in relations with the United States, even as President Barack Obama and China’s President Xi Jinping have sought to deepen cooperation in other areas, such as climate change.

Pentagon spokesman Brent Colburn said the U.S. was aware of the artillery, but he declined to provide other details. Defense officials described the weapons as self-propelled artillery vehicles and said they posed no threat to the U.S. or American territories.

While 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.”

“Turning an underwater rock into an airfield simply does not afford the rights of sovereignty or permit restrictions on international air or maritime transit,” Carter told the audience at the International Institute for Strategic Studies summit.

China’s actions have been “reasonable and justified,” said Senior Col. Zhao Xiaozhuo, deputy director of the Center on China-America Defense Relations at the People’s Liberation Army’s Academy of Military Science.

Zhao challenged 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.

Carter responded that China’s expanding land reclamation projects are unprecedented in scale. He said the U.S. has been flying and operating ships in the region for decades and has no intention of stopping.

While 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 Carter is scheduled to visit during this 11-day trip across Asia. Others are Malaysia, the Philippines and Taiwan.

Asked about images of weapons on the islands, China’s Foreign Ministry spokeswoman Hua Chunying said she was “not aware of the situation you mention.”

She also scolded Carter, saying the U.S. should be “rational and calm and stop making any provocative remarks, because such remarks not only do not help ease the controversies in the South China Sea, but they also will aggravate the regional peace and stability.”

Carter appeared to strike back in his speech, saying that the U.S. is concerned about “the prospect of further militarization, as well as the potential for these activities to increase the risk of miscalculation or conflict.” And he said the U.S. “has every right to be involved and be concerned.”

But while Carter stood in China’s backyard and added to the persistent drumbeat of U.S. opposition to Beijing’s activities, he did little to give Asia-Pacific nations a glimpse into what America is willing to do to achieve a solution.

He said the U.S. will continue to sail, fly and operate in the region, and warned that the Pentagon will be sending its “best platforms and people” to the Asia-Pacific. Those would include, he said, new high-tech submarines, surveillance aircraft, the stealth destroyer and new aircraft carrier-based early-warning aircraft.

U.S. Sen. John McCain, R-Ariz., who also is attending the Singapore conference, told reporters that the U.S. needs to recognize that China will continue its activities in the South China Sea until it perceives that the costs of doing so outweigh the benefits.

He said he agreed with Carter’s assertion that America will continue flights and operations near the building projects, but “now we want to see it translated into action.”

One senior defense official has said the U.S. is considering more military flights and patrols closer to the projects in the South China Sea, to emphasize reclaimed lands are not China’s territorial waters. Officials also are looking at ways to adjust the military exercises in the region to increase U.S. presence if needed. That official was not authorized to discuss the options publicly and spoke on condition of anonymity.

One possibility would be for U.S. ships to travel within 12 miles of the artificial islands, to further make the point that they are not sovereign Chinese land. McCain said it would be a critical mistake to recognize any 12-mile zone around the reclamation projects.

The U.S. has been flying surveillance aircraft in the region, prompting China to file a formal protest.

U.S. and other regional officials have expressed concerns about the island building, including worries that it may be a prelude to navigation restrictions or the enforcement of a possible air defense identification zone over the South China Sea. China declared such a zone over disputed Japanese-held islands in the East China Sea in 2013.

China has said the islands are its territory and that the buildings and other infrastructure are for public service use and to support fishermen.