U.S. surveillance imagery shows China has positioned weaponry on one of the artificial islands it is developing in the South China Sea, American officials said, supporting their suspicions that Beijing has been building up reefs for military purposes.

The U.S. imagery detected two Chinese motorized artillery pieces on one of the artificial islands built by China about one month ago. While the artillery wouldn’t pose a threat to U.S. planes or ships, U.S. officials said it could reach neighboring islands and that its presence was at odds with China’s public statements that the reclaimed islands are mainly for civilian use.

“There is no military threat,” a U.S. official told The Wall Street Journal on Thursday. “But it is about the symbolism.”

While posing no military threat to the U.S., the motorized artillery was within range of an island claimed by Vietnam that Hanoi has armed with various weaponry for some time, the American officials said. Vietnamese officials didn’t immediately respond to a request for comment.

A Chinese Embassy spokesman in Washington wouldn’t comment specifically on the weaponry, but said its development work within the Spratly Islands—known by the Chinese as the Nansha Islands—was primarily civilian.

“It needs to be emphasized that the Nansha Islands is China’s territory, and China has every right to deploy on relevant islands and reefs necessary facilities for military defense,” said Zhu Haiquan, the spokesman for the Chinese embassy. “However, the facilities on relevant islands and reefs are primarily for civilian purposes.”

Officials from the U.S., China and across Asia are gathering in Singapore for the annual Shangri-La Dialogue, one of the most important regional security conference of the year. Washington and Beijing have been in an escalating dispute over China’s artificial island construction in the South China Sea, compounding earlier disputes over China’s aggressive claims in the area.

Lt. Col. Jeff Pool, a Pentagon spokesman traveling in Asia with Defense Secretary Ash Carter, declined to comment on the issue. Mr. Carter has called for “a lasting halt” to the land reclamation by China and others in the South China Sea, noting that the U.S., unlike China, is not staking any land claims in the region, but is seeking stability.

The U.S. has been flying Navy surveillance planes near the artificial islands and has monitored them using reconnaissance satellites. U.S. and allied officials previously have said China has built radar installations and landing strips on the artificial islands.

U.S. officials said they first detected the artillery pieces on an island about a month ago. “We’ve been aware of these for a little while,” a senior U.S. official said.

American officials said that the equipment more recently has either been removed or purposely obscured from view by the Chinese. It was unclear how or why the equipment was no longer visible.

Mr. Zhu, the Chinese embassy spokesman, said China’s activities on the islands and reefs “are aimed at first and foremost improving the working and living conditions for personnel stationed there and better fulfilling China’s relevant international responsibilities and obligations,” he said.

China has been critical both of the U.S. rhetoric as well as flights by American surveillance planes near the islands. Ministry of Foreign Affairs spokeswoman Hua Chunying told reporters Thursday that the U.S. made “inappropriate remarks about China’s construction work.” Building the islands are within the sovereign rights of China, she said.

Ms. Hua and other Chinese officials assert that Washington has a double standard, criticizing the U.S. for being “selectively mute” about construction activities carried out by other countries in the region.

She didn’t name those claimants, but Vietnam, Taiwan, Malaysia and the Philippines have also engaged in reclamation and other work in areas they control.

“If it’s not a double standard, then there must be some hidden motives behind that,” Ms. Hua said at a daily media briefing.

U.S. officials said the reclamation efforts by those countries are on a far smaller scale than China, which Washington says has dramatically stepped up its construction efforts this year. The Pentagon confirmed earlier this month that Beijing has built a total of 2,000 acres of new land mass across seven islands. About 1,500 acres of that total were created since January, defense officials have said.

John Kirby, the State Department spokesman, declined to comment on the surveillance pictures, but said that Secretary of State John Kerry had given a clear warning to China.

“We watch every outpost in the South China Sea carefully and monitor the actions of the countries concerned,” Mr. Kirby said in a statement. “I won’t speak to intelligence issues, but I can assure you that Secretary Kerry is in touch with the claimants, including China, and has been very clear in warning against actions that escalate tensions. We oppose upgrades or militarization of outposts in disputed areas of the South China Sea. We ask all claimants to show restraint and halt reclamation in favor of diplomacy.”

In Hawaii on Wednesday, Mr. Carter said China was isolating itself from the region by its actions in the South China Sea and reiterated the U.S. position that American aircraft and Navy vessels would navigate on the water and in the air wherever international law allowed.

The U.S. assertion of navigation and air flight rights, coupled with China’s vow to defend what it sees as its sovereignty around the islands, adds up to a potential confrontation between the two powers.

Mr. Carter told reporters that despite Beijing’s arguments, China is responsible for any instability.

“The new facts are not created by the United States,” Mr. Carter said as his plane flew into Singapore. “The new facts are created by China.”

—Te-Ping Chen contributed to this article.