BEIJING (Reuters) - China has returned a U.S. underwater drone taken by one of its naval vessels in the disputed South China Sea last week after what it said were friendly talks with the United States, which reiterated its criticism of the “unlawful” seizure.

The oceanographic survey ship, USNS Bowditch, is shown September 20, 2002, which deployed an underwater drone seized by a Chinese Navy warship in international waters in South China Sea, December 16, 2016. Courtesy U.S. Navy/Handout via REUTERS

The taking of the unmanned underwater vehicle in international waters near the Philippines triggered a diplomatic protest and speculation about whether it would strengthen U.S. President-elect Donald Trump’s hand as he seeks a tougher line with China.

A Chinese naval ship took the drone, which the Pentagon says uses unclassified, commercially available technology to collect oceanographic data, on Thursday about 50 nautical miles northwest of Subic Bay in the Philippines.

China’s defense ministry said in a brief statement the drone had been given back to the United States on Tuesday.

“After friendly consultations between the Chinese and U.S. sides, the handover work for the U.S. underwater drone was smoothly completed in relevant waters in the South China Sea at midday,” the ministry said.

The defense ministry declined to give more details about the handover when contacted by Reuters.

The Pentagon said the vehicle had been handed over to the guided missile destroyer USS Mustin near where it had been “unlawfully seized”. It called on China to comply with international law and refrain from further efforts to impede lawful U.S. activities.

“The U.S. remains committed to upholding the accepted principles and norms of international law and freedom of navigation and overflight and will continue to fly, sail, and operate in the South China Sea wherever international law allows,” Pentagon Press Secretary Peter Cook said in a statement.

Chinese foreign ministry spokeswoman Hua Chunying referred questions about the handover and other details of the case to the defense ministry.

“The handling of this incident shows that the Chinese and U.S. militaries have quite smooth communication channels. We think that this communication channel is beneficial to timely communication and the handling of sudden incidents and prevention of miscalculations and misunderstandings,” she said.

“As to what the U.S. defense department said, I have to verify it with the military. But I think what they said is unreasonable as we have always said that for a long time the U.S. military has regularly sent ships and aircraft to carry out close up surveillance and military surveys in waters facing China, which threatens China’s sovereignty and security,” Hua told reporters.

“China is resolutely opposed to this and has always demanded the U.S. end these kinds of activities. I think this is the cause of this or similar incidents.”

The seizure has added to U.S. concern about China’s growing military presence and aggressive posture in the disputed South China Sea, including its militarization of maritime outposts.

China is deeply suspicious of any U.S. military activity 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 U.S. Navy has about 130 such underwater drones, made by Teledyne Webb, each weighing about 60 kg (130 lb) and able to stay underwater for up to five months. They are used around the world to collect unclassified data about oceans, including temperature and depth.

It is not clear how many are used in the South China Sea.