BEIJING — China on Tuesday handed back to the U.S. an underwater drone whose seizure raised tensions in a relationship that has been tested by President-elect Donald Trump's signals of a tougher policy toward Beijing.

Last week's incident near the Philippines prompted a new volley of criticism by Trump, who has riled the Chinese leadership by saying he might reconsider policy toward Taiwan, the self-ruled island the mainland claims as its territory.

We should tell China that we don't want the drone they stole back.- let them keep it! — Donald J. Trump (@realDonaldTrump) December 18, 2016

The Chinese navy vessel that seized the drone returned it near where it had been taken, and it was received by the USS Mustin about 50 miles northwest of Subic Bay in the Philippines, Pentagon press secretary Peter Cook said in a statement. Cook said Washington considered the seizure illegal.

"This incident was inconsistent with both international law and standards of professionalism for conduct between navies at sea," he said, adding that the U.S. has called on China to refrain from "further efforts to impede lawful U.S. activities."

The statement said the U.S. would continue to "fly, sail, and operate in the South China Sea wherever international law allows." Such freedom of navigation missions in which U.S. ships sail near China's artificial islands draw warnings and rebukes from Beijing.

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China's defense ministry said in a statement that it handed the drone back after "friendly consultations." The country's foreign ministry added Tuesday that U.S. criticism of the drone issue was unreasonable.

Chinese officials say the drone was removed from the water to ensure the safety of passing ships, but domestic political experts have read the move as a warning to Trump not to test Beijing's resolve over Taiwan.

The incident underscores how Trump will confront as president an increasingly assertive China that wants to extend its reach in the South China Sea, a strategically vital area through which about $5 trillion in global trade passes each year. Several of China's smaller neighbors have protested China's territorial claims there and are closely watching Trump's handling of the disputed sea.

The U.S. said the drone was being operated by civilian contractors in international waters. A noncombat ship was recovering two drones last Thursday when a Chinese ship approached, launched a small boat and picked up one of them, officials say.

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The U.S. Navy has about 130 such underwater drones, made by Teledyne Webb, each weighing about 130 pounds and able to stay underwater for up to five months. They are used to collect unclassified data about oceans, including temperature and depth.

The USNS Bowditch CHINFO, Navy Visual News / via AP

China is deeply suspicious of any U.S. military activities in the resource-rich South China Sea, with state media and experts saying the use of the drone was likely part of U.S. surveillance efforts in the disputed waterway.

The overseas edition of the ruling Communist Party's People's Daily said in a commentary on Monday the USNS Bowditch, which was fielding the drone and was set to pick it up, was a "serial offender" when it came to spying operations against China.