A Chinese Navy ship stole a U.S. Navy unmanned underwater vehicle — a type of drone — from international waters in the South China on Thursday, according to the Pentagon.

USNS Bowditch U.S. Navy

Two U.S. Navy Ocean Glider Unmanned Underwater Vehicle were collecting data during what the Pentagon called a routine survey approximately 50 nautical miles north west of Subic Bay, in the Philippines. The incident did not take place near the hotly disputed Spratly Islands.

The gliders “stopped dead in the water” as they awaited recovery by the USS Bowditch, an oceanographic research vessel. A Chinese ship that was already shadowing the Bowditch swooped in and “unlawfully retrieved” one of the gliders.

“They stole it,” an official with knowledge of the incident told NBC News.

There were multiple attempts at bridge to bridge communication to demand the Chinese give the glider back. The Chinese acknowledged the call but said that they were returning to normal operations, ignored requests to return the glider and left the area.

“This is an action that’s not in keeping with international law,” Pentagon spokesperson Captain Jeff Davis said on Friday.

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The issue is being addressed through diplomatic channels, the official said, adding that the U.S. has already issued a formal demarche, or messaged request to China.

A defense official tells NBC News that the U.S. Navy knows where the Chinese ship is, but are not actively tracking it.

The officials said this does not likely qualify as an act of war.

“No one here is freaking out about this, but it was unprofessional,” the official said.

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This glider is commercially available and they retail for about $150,000. These types of gliders are generally used for environmental collection to help the Navy better understand the oceans and how sonar works. They are also used to help the Navy better conduct anti-submarine warfare

The stolen glider belongs to Military Sealift Command and was not conducting surveillance.

The incident follows a series of tense exchanges with China, including that nation's sharp rebuke of president-elect Donald Trump's phone call with Taiwan's president in a departure from decades-long policy toward the island prefecture.

Two Chinese military aircraft intercepted a U.S. military reconnaissance plane over the South China Sea in May. And in October, a U.S. destroyer sailed near the artificial islands built by China in the disputed waters of the South China Sea in a move that was seen as an open challenge to Beijing's territorial claims.