The Pentagon is demanding that China return an unmanned underwater U.S. Navy vehicle it seized in the South China Sea.
The department said in a statement Friday it is using "appropriate government-to-government channels," to call upon China to return the drone "immediately."
"We call upon China to return our [unmanned, underwater vehicle or UUV] immediately, and to comply with all of its obligations under international law," Pentagon press secretary Peter Cook said.
"The UUV is a sovereign immune vessel of the United States," he added.
The incident occurred Thursday around noon local time in international waters off the coast of the Philippines, according to a defense official.
The USNS Bowditch, an oceanographic survey ship, was preparing to retrieve its unmanned drone out of the water, as part of their typical mission to collect data on the ocean and weather patterns, the official said. The drone had surfaced and sent out a signal as to its location per normal operations.
A Chinese ship that had been shadowing the Bowditch then dropped its own small boat in the water and swooped in to grab the drone, the official said.
The Bowditch crew then called over via radio to the Chinese ship to ask for their equipment back. The Chinese crew confirmed receipt of the message, but began sailing away, leaving with the drone.
Around noon local time on Friday, the U.S. State Department filed an official demarche with China. The official said the matter is now in the State Department's hands.
"It's important that we want to note that this drone is sovereign property of the United States that was obtained unlawfully," the official told The Hill.
The defense official described the Bowditch's operation as "completely normal."
"They do these sort of survey operations, collect data on the ocean and other weather patterns," the official said.
In his statement Friday, Cook said the Bowditch and the drone were conducting "routine operations in accordance with international law" about 50 nautical miles northwest of Subic Bay.
The drone seized Thursday is an unclassified "ocean glider" system used around the world to gather data such as salinity, water temperature and sound speed, he said.
China considers the South China Sea its sovereign territory and has built islands out of reef features in those waters. The sea is also claimed by many other countries.
The U.S. Navy has operated in international waters in the area to ensure freedom of navigation for all countries, at times sending ships and flying aircraft through it.
This story was last updated at 1:30 p.m.