Story highlights One of two underwater drones was seized by a Chinese naval ship

The drone was simply measuring ocean conditions, a US official told CNN

(CNN) The Pentagon is demanding that China return an "unlawfully seized" underwater drone after a Chinese warship took the device from waters near a US oceanographic vessel.

"We call upon China to return our UUV immediately, and to comply with all of its obligations under international law," Pentagon press secretary Peter Cook said in a statement, using the abbreviation for "unmanned underwater vehicle."

In the latest encounter in international waters in the South China Sea region, the USNS Bowditch was sailing about 100 miles off the Philippine port at Subic Bay when the incident occurred, according to the official.

Bowditch had stopped in the water to pick up two underwater drones. At that point a Chinese naval ship that had been shadowing the Bowditch put a small boat into the water. That small boat came up alongside and the Chinese crew took one of the drones.