SINGAPORE — Two large artillery vehicles were detected on one of the artificial islands that China is creating in the South China Sea, U.S. officials said Friday, underscoring ongoing concerns that Beijing may try to use the land reclamation projects for military purposes.

The discovery was made at least several weeks ago by the United States, but it's not clear if the weapons are still there or if they have been moved or hidden out of sight, officials said.

The revelation comes as Defense Secretary Ash Carter begins an 11-day trip, including several stops in the Asia Pacific. He is slated to speak Saturday at an international security summit here, and is expected to reassert America's views that China and other nations must stop all land reclamation projects in the region.

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Pentagon spokesman Brent Colburn said the U.S. was aware of the artillery, but he declined to provide other details, saying it is 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 so spoke on condition of anonymity.

The sighting was first reported by the Wall Street Journal.

U.S. officials have been watching the rapidly expanding land reclamation by China, which, according to estimates, totals more than 2,000 acres in the South China Sea.

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

Carter on Wednesday made it clear that the U.S. will "fly, sail and operate wherever international law allows." But so far he has said little about what the U.S. is willing to do to get China to stop the island construction.

Asked about the latest imagery suggesting China had put weapons on one of the land reclamation islands, China's Foreign Ministry spokeswoman Hua Chunying said she is "not aware of the situation you mention, but China has clearly reiterated its position several times on the islands in the South China Sea."

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