US officials say a Chinese nuclear submarine has sunk while docked

A Chinese nuclear submarine sank in its dock during construction earlier this year, a major setback for the Chinese military, according to U.S. defense officials.

Speaking to the BBC's US partner CBS on condition of anonymity, the officials said the loss of the submarine likely occurred between May and June.

Satellite images taken in June show what appear to be floating salvage cranes at the berth in Wuhan, where the ship was spotted a month earlier. Beijing has not confirmed the reports.

The U.S. officials said it was “not surprising” that the Chinese military covered up the loss of one of its newest assets.

It is unclear whether the submarine was carrying nuclear fuel at the time.

The BBC has reviewed commercial satellite images collected from the Wuhan shipyard. Images taken on June 15 appear to show a number of crane vessels operating in and around the waters next to a berth.

Previous images from March 7 and May 29 show a single ship docked at the same berth.

The comparatively poor quality of these images makes it difficult to conclude whether the two previous events are in any way related and, crucially, whether they actually show a specific submarine at berth.

The BBC showed these images to Mike Plunkett, a marine expert from Janes, who agreed they offered limited insight.

He explained that the images seen on May 29 “could show a submarine with an X-shaped rudder arrangement.”

However, this alone was not enough to confirm that it was a Type 041, as it could also be a modified Type 039.

He added: “It would not be unusual to see a submarine next to this pontoon,” as China has one of its main conventional submarine production sites in Wuhan.

A Chinese Foreign Ministry spokesman said Friday he was not familiar with the issue and did not provide any information when asked about it at a news conference in Beijing.

The incident raises questions about China's defense industry, which is said to be rife with corruption.

China has the largest navy in the worldwith more than 370 ships, and is currently producing a new generation of nuclear-armed submarines, the Zhou class, of which this was the first.

Taiwan said it had conducted its own investigation into the submarine's fate and “understood the situation through multiple intelligence and surveillance methods,” but did not provide further details.

Thomas Shugart, a former U.S. Navy submariner and analyst at the Center for a New American Security, first noticed the submarine incident in July.

He told the BBC the sinking was a “setback” that would cause “pretty big embarrassment” for the People's Liberation Army (PLA) Navy, but the security risk was probably “pretty low.”

However, he added: “When this ship is eventually repaired, and I'm sure it will be, it will be a far more capable submarine than what they have previously built at this shipyard.”

“I don’t think it will significantly alter the really impressive upward trajectory of the PLA Navy’s capabilities.”

The demise comes at a time when Beijing is increasingly asserting its claim virtually the entire South China Seawhich is crucial for international trade.

There are longstanding maritime disputes with other nations in the region, including Brunei, Malaysia, the Philippines, Taiwan and Vietnam.