U.S. officials say that two large artillery vehicles have been detected on one of the artificial islands that China is creating in the South China Sea.

The discovery, made at least several weeks ago by the United States, underscores ongoing concerns that China will try to use the land reclamation projects for military purposes.

Pentagon spokesman Brent Colburn says the U.S. was aware of the artillery, but he declined to provide other details, saying it is an intelligence matter. The revelation comes as Defense Secretary Ash Carter visits several Asia Pacific nations.

Another defense official described the weapons as self-propelled artillery vehicles. The official was not authorized to discuss the matter publicly so spoke on condition of anonymity.

The sighting was first reported by the Wall Street Journal.