SINGAPORE — The United States has spotted a pair of mobile artillery vehicles on an artificial island that China is building in the South China Sea, a resource-rich stretch of ocean crossed by vital shipping lanes, American officials said.

China’s construction program on previously uninhabited atolls and reefs in the Spratly Islands has already raised alarm and drawn protests from other countries in the region, whose claims to parts of the South China Sea overlap with China’s. The United States has also become increasingly vocal about its objections in recent days.

Defense Secretary Ashton B. Carter called this week for China to halt the construction, saying that international law did not recognize Chinese claims of sovereignty over the new territories and that American warships and military aircraft would continue to operate in the area. He reiterated those assertions in a speech Saturday morning in Singapore at the Shangri-La Dialogue, an annual security conference that brings together most Asian countries and many other powers outside the region.

“We all know that turning an underwater rock into an airfield simply does not afford the rights of sovereignty,” Mr. Carter said.