BEIJING, China (AP) — China said yesterday that its military seized a US Navy unmanned underwater glider in the South China Sea to ensure the “safe navigation of passing ships” in one of the most serious incidents between the two militaries in years.

The Chinese Navy on Thursday seized the drone, which the Pentagon said was being operated by civilian contractors to conduct oceanic research. The US said it issued a formal diplomatic complaint over the seizure and demanded the drone’s return.

Chinese Defense Ministry spokesman Yang Yujun issued a statement late yesterday, saying that a Chinese Navy lifeboat discovered an unknown device in the South China Sea on Thursday. “In order to prevent this device from posing a danger to the safe navigation of passing ships and personnel, the Chinese lifeboat adopted a professional and responsible attitude in investigating and verifying the device,” Yang said.

The statement said that after verifying that the device was an American unmanned submerged device, “China decided to transfer it to the US through appropriate means”.

The statement also accused the US of long deploying ships “in China’s presence” to conduct “military surveying.”

“China is resolutely opposed to this and requests the US stop such activities,” it said. “China will continue to maintain vigilance against the relevant US activities and will take necessary measures to deal with them.”

Yesterday China’s foreign ministry said the country’s military was in contact with its American counterparts on “appropriately handling” the incident, though it offered no details on what discussions were underway.

The drone was seized while collecting unclassified scientific data about 92 kilometres (57 miles) northwest of Subic Bay near the Philippines in the South China Sea, which China claims virtually in its entirety, Navy Captain Jeff Davis, a Pentagon spokesman, said Friday.

“It is ours. It’s clearly marked as ours. We would like it back, and we would like this not to happen again,” Davis told reporters. He said the drone costs about $150,000 and is largely commercial, off-the-shelf technology.

The USNS Bowditch, which is not a combat ship, was stopped in international waters Thursday afternoon while recovering two of the gliders when the Chinese ship approached, Davis said. The two vessels were within about 450 metres of each other. He said that the USNS Bowditch carries some small arms, but that no shots were fired.

According to the Pentagon, as the Chinese ship left with the drone, which is about 3 metres long, its only radio response to the US vessel was, “We are returning to normal operations.”

President-elect Donald Trump blasted the seizure, apparently misspelling “unprecedented,” and tweeted yesterday: “China steals United States Navy research drone in international waters — rips it out of water and takes it to China in unpresidented act.”

He later reissued the tweet, correcting the spelling to “unprecedented.”

Last weekend, Trump was criticised on social media for bad spelling in a tweet in which he accused CNN of reporting “rediculous” fake news. Hours later he put out a fresh tweet correcting the spelling to “ridiculous”.

Bonnie Glaser, senior advisor for Asia at the Centre for Strategic and International Studies, said the seizure of the glider occurred inside the exclusive economic zone of the Philippines, not China, and appeared to be a violation of international law.

China delineates its South China Sea claims with a roughly drawn sea border known as the “nine-dash line” that runs along the west coast of the Philippines. However, it hasn’t explicitly said whether it considers those waters as sovereign territory, and says it doesn’t disrupt the passage of other nations’ shipping through the area. The US doesn’t take a position on sovereignty claims, but insists on freedom of navigation, including the right of its naval vessels to conduct training and other operations in the sea.

Davis said that the incident could be the first time in recent history that China has taken a US naval vessel. Some observers have called it the most significant dispute between the sides’ militaries since the April 2001 mid-air collision between a US Navy surveillance aircraft and a Chinese fighter jet about 110 kilometres from China’s Hainan island that led to the death of a Chinese pilot.