China on Tuesday returned an underwater U.S. drone it seized last week in the international waters of the South China Sea.
Pentagon spokesman Peter Cook said the transfer took place in the same area where China picked up the drone, which measures water salinity and temperature gradients.
"This incident was inconsistent with both international law and standards of professionalism for conduct between navies at sea," Cook said in a statement. "The U.S. has addressed those facts with the Chinese through the appropriate diplomatic and military channels, and have called on Chinese authorities to comply with their obligations under international law and to refrain from further efforts to impede lawful U.S. activities."
China's Defense Ministry said Tuesday the transfer was conducted "smoothly" after "friendly consultations" between the two countries.
Foreign Ministry spokeswoman Hua Chunying later said the U.S. regularly conducts close up surveillance in China's coastal waters, which China strongly opposes.
China has said the situation began when one of its vessels saw a piece of "unidentified equipment" in the water and retrieved it for the sake of "navigational safety."
The incident raised fresh concerns in Washington and among Asian allies about China's stepped-up military presence in the South China Sea, and what critics call Beijing's aggressive responses to competing maritime claims in the region.
Brunei, the Philippines, Vietnam and Malaysia all claim mineral and fishing rights in the region, and many of those claims overlap.
While Washington has taken no official stance on the rival sovereignty claims, the U.S. Navy has sought to protect the region's crowded international shipping lanes. The Obama administration has also challenged China's efforts to boost its maritime presence with military hardware deployed in areas claimed by other regional governments.