Defense Secretary Ash Carter on Saturday urged China to stop trying to convert artificial reefs in the South China Sea into a military airfield but also made clear the U.S. has no intentions of ending air-and-sea operation in that region.

Carter made his comment at an international security conference filled with Asia-Pacific leaders and also said the United States has been flying and operating ships in the region for decades and opposes “any further militarization” of the disputed lands.

He also said the reclamation project is out of step with international rules and that turning underwater land into airfields won’t expand Beijing’s sovereignty.

A Chinese military officer in the crowd immediately slammed Carter’s comments as “groundless and not constructive.”

Arizona GOP Sen. John McCain, chairman of the Senate Armed Services Committee, who also is attending the Singapore conference, 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."

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He also 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.

Defense Department officials said they discovered several weeks ago that China had put two large artillery vehicles on one of the islands, inciting fears that the reefs will be for military purposes.

However, the Pentagon will not release photos to support its contention that the vehicles were there.

One senior defense official told The Associated Press that 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 are also looking at ways to adjust military exercises in the region to increase U.S. presence if needed.

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.

China’s behavior in the South China Sea has become a sore point in relations with the U.S., even as President Obama and China’s President Xi Jinping have worked to deepen cooperation in other areas, such as climate change.

Pentagon spokesman Brent Colburn said the U.S. was aware of the artillery but declined to provide any 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.

Although Carter directed most of his criticism toward China, he urged other nations who are doing smaller land reclamation projects to stop.

Carter is set to meet with one of those countries, Vietnam, during his 11-day tour 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 responded saying 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.” Carter also maintained 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.

Carter 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. 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.

The Associated Press contributed to this report.