Steph Solis

USA TODAY

After what it described as "friendly consultations," China's Defense Ministry said it returned a U.S. Navy underwater drone that was seized last week in the South China Sea.

The drone, conducting "routine operations" in international waters, was taken by the Chinese Navy on Thursday, 50 nautical miles northwest off Subic Bay in the Philippines.

Pentagon spokesman Peter Cook said in a statement that officials made clear that the drone was unlawfully seized and urged Chinese authorities not to interfere with lawful U.S. operations.

"This incident was inconsistent with both international law and standards of professionalism for conduct between navies at sea," Pentagon spokesman Peter Cook said in a statement. "The U.S. has addressed those facts with the Chinese through the appropriate diplomatic and military channels."

China's Defense Ministry spokesman, Yang Yujun, said in a statement that a navy lifeboat found the unknown device on Thursday. China kept it to ensure the "safe navigation of passing ships" as authorities verified that it was an American drone.

The drone had just finished a military oceanographic survey route and was programmed to return to the USNS Bowditch, a Navy survey ship, according to the Pentagon.

Before the drone's capture, U.S.-China relations were already tense over the presidential election of billionaire Donald Trump, who promised during his campaign to get tough on Chinese trade. Earlier this month, Trump took a call with Taiwan President Tsi Ing-wen — a first by a U.S. president or president-elect since President Jimmy Carter acknowledged in 1979 China's position that Taiwan is part of “One China.”

Trump accused China of stealing the drone in a tweet last week.

Contributing: Heidi M. Przybyla and Jim Michaels, USA TODAY; The Associated Press.