WASHINGTON — The Pentagon said on Friday that it had issued a formal protest to Beijing demanding the return of an underwater drone seized by a Chinese warship in the South China Sea, an incident that risked increasing tensions in a region already fraught with great-power rivalries.
A Defense Department official said that the unmanned underwater vehicle was discovered missing on Thursday when the crew of the United States Navy vessel Bowditch tried to retrieve it.
The Bowditch, an oceanographic ship, was operating in international waters and carrying out scientific research, said the official, who spoke on the condition of anonymity to describe a potentially delicate international incident.
American officials said they were still trying to determine whether the seizure was a low-level action taken by Chinese sailors who spotted the drone or a strategic-level action ordered by more senior Chinese leaders.
Whatever the case, the Pentagon said that China had no right to seize the drone.
“This is not the sort of conduct we expect from professional navies,” said Capt. Jeff Davis, a Pentagon spokesman.
The incident is the second this week in the South China Sea, coming just after China signaled on Thursday that it had installed weapons on disputed islands that it would use to repel threats. The latest moves complicate already testy relations between China and the United States, ties that have been further complicated by President-elect Donald J. Trump’s phone call with the president of Taiwan.
Mr. Trump angered Chinese officials by holding a phone conversation with President Tsai Ing-wen of Taiwan, an island that Beijing deems a breakaway province of China. It had been nearly four decades since a United States president or president-elect had such direct contact with a Taiwanese leader.
In an interview broadcast on Sunday, Mr. Trump also criticized China over its trade imbalance with the United States, its military activities in the South China Sea and its ties to North Korea.
Defense officials said on Friday that they were trying to determine if the seizure of the underwater drone had anything to do with Mr. Trump’s comments.