A Chinese naval ship seized an unmanned ocean glider that was being used by the U.S. Navy to test water conditions in international waters in the South China Sea, the Pentagon said Friday.

The glider was about 500 yards from the USNS Bowditch, an oceanographic survey vessel with a mostly civilian crew. The U.S. ship contacted the Chinese ship and asked for the glider, which collects unclassified data, but the Chinese ship, which had brought the glider aboard, left the area.

“The United States has through their proper diplomatic channels demarched the Chinese, demanding return of our stuff,” a defense official said.

The incident occurred about 24 hours ago, the official said.

The Navy has used “Seaglider” underwater drones for years to collect sea samples, sending the craft between waypoints using sensors and satellite communications. They typically travel just a few miles per hour, and are tracked by oceanographic vessels like the USNS Bowditch that was involved Thursday.

Read more from The Washington Post.