SINGAPORE >> U.S. surveillance detected two large artillery vehicles on one of the artificial islands that China is creating in the South China Sea, U.S. officials said Friday, heightening concerns that Beijing could use the land reclamation projects for military purposes.

The revelation came as Defense Secretary Ash Carter was in the region for an international security summit in Singapore where he is expected to demand anew that China and other nations halt all such projects. While scolding China for aggression, Carter is not expected to offer any indication of what the U.S. might do if the projects proceed.

The weaponry was discovered at least several weeks ago, and it’s not clear if it is still there or may have been moved or hidden, officials said. The Pentagon would not release any photos to support its contention.

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, saying it was an intelligence matter. Defense officials described the weapons as self-propelled artillery vehicles and said they posed no threat to the U.S. or American territories. The officials were not authorized to discuss the matter publicly and spoke only on condition of anonymity.

The sighting was first reported by The Wall Street Journal.

Mira Rapp Hooper, director of the Asia Maritime Transparency Initiative, which monitors developments in the South China Sea, said that analysts have previously identified artillery on at least two of the Chinese land reclamation sites in the Spratly Island chain: Fiery Cross, where an airstrip is under construction, and Gaven Reef. The transparency initiative is a project of the Washington-based Center for Strategic and International Studies

U.S. officials have been watching the rapidly expanding land reclamation by China, which is estimated to total more than 2,000 acres in the South China Sea. In its annual report on China’s military power earlier this month, the Pentagon warned that five emerging outposts could be used for surveillance systems, harbors, an airfield and logistical support.

The U.S. has been flying surveillance aircraft in the region, prompting China to file a formal protest after a Navy P-8A Poseidon recently flew over one of the sites.

In the past year, the U.S. has escalated its criticism of China’s claim to virtually all of the resource-rich South China Sea, saying it is unsupported by international law. Beijing’s expansive claims to the waters and reefs overlap those of the Philippines, Taiwan, Brunei, Malaysia and Vietnam.

While the U.S. is not a claimant, it says it has a national interest in a peaceful resolution of the territorial disputes and freedom of navigation. Last June, the U.S. called for a freeze on construction work in disputed areas, but Beijing only increased its land reclamation. In recent months, commercial satellite imagery has put a spotlight on the rapid expansion of artificial islands, mostly located in the Spratlys.

Since early this year, U.S. lawmakers have been raising alarms — displaying pictures of the land reclamation at congressional hearings and calling for a more robust U.S. response to actions that pose a challenge to decades of American predominance in the Asia-Pacific.

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.”

China has said the islands are its territory and that the buildings and other infrastructure are for public service use and to support fishermen. It accuses the Philippines, Vietnam and others of carrying out their own building work on other islands.

In a speech at Pearl Harbor on Wednesday, Carter made it clear that the U.S. will “fly, sail and operate wherever international law allows.” But beyond the tough rhetoric, there are no clear answers to how far the U.S. might be willing to go to stop China’s island construction.

The U.S. and its Pacific allies have expressed concerns that the island building might be a prelude to navigation restrictions or the enforcement of an 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.

Carter on Friday flew over the crowded Straits of Malacca and Singapore, in part to emphasize a need for continued freedom of navigation in the region.

On board two V-22 Ospreys, Carter and his staff and several members of the media flew over the narrow shipping lanes, which were packed with massive container ships and other vessels.

The busy waterway is “a very striking example of the link between security and prosperity and the importance of having security and stability in the Pacific,” said Kelly Magsamen, the Pentagon’s principle deputy assistant secretary for Asia Pacific matters.

The Malacca Strait is 550 miles long, but just 1.7 miles wide at its narrowest point. About a third of global shipping moves through the strait — or about 50,000 ships a year. Any blockage of the strait would force ships to switch to longer and more expensive routes.

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Pennington reported from Washington. AP news assistant Liu Zheng in Beijing contributed to this report.