Global Hawk long-range surveillance drones were targeted by jamming in at least one incident near the Spratly Islands, where China is building military facilities on Fiery Cross Reef, the Washington Free Beacon reported.
That statement follows Thursday reports that the Chinese navy warned a US surveillance plane to leave the same area eight times in an apparent effort to establish and enforce a no-fly zone, a demand Washington rejected.
"This is the Chinese navy … This is the Chinese navy … Please go away … to avoid misunderstanding," a radio call in English from an installation on Fiery Cross said. The warnings were reported by CNN, which had a crew on the aircraft.
Pentagon spokesman Colonel Steve Warren said the United States does not recognize China's sovereignty claims over the new islands. He added that flights and Navy ships will continue their routine patrols, but will maintain a distance of at least 12 miles from the island.
Details of the drone interference are classified, but last week, David Shear, the assistant defense secretary for Asian and Pacific security affairs, said Global Hawks are deployed in Asia as one element of a buildup of forces near the South China Sea.
"We're engaged in a long-term effort to bolster our capabilities in the region,” Shear told the Senate Foreign Relations Committee. "Just a few examples of the increases in our capabilities in the region include the deployment of Global Hawks and F-35s. Soon we will be adding to the stock of V-22s in Japan as well."
Shear said the Pentagon estimates that China will complete construction of an airfield on Fiery Cross Reef by 2017 or 2018. Meanwhile, rapid militarization has security experts worried about the potential for a conflict.
Rick Fisher, a China military affairs analyst, said China could increase pressure on the United States to halt surveillance flights in Asia by first attacking one of the unmanned aircraft flights.
"Though UAVs like the Global Hawk are rather expensive, they are also regarded as more expendable because they are unmanned," Fisher, a senior fellow at the International Assessment and Strategy Center, told the Washington Free Beacon.
"But failing to defend these UAVs runs the risk of China viewing them as 'fair game' to shoot down whenever they please."
Beijing also might attempt to capture a Global Hawk by causing one to crash in shallow water, or by attempting to snatch one in flight using a manned aircraft, Fisher said.