In a major setback to China's efforts in expanding its maritime military power, its newest state-of-the-art nuclear-powered submarine sank in a dock, US officials said.

The incident took place sometime in May or June this year at the Wuchang shipyard near Wuhan. Despite the Beijing's serious efforts to cover-up the matter, satellite imagery has brought the incident to light.

With over 370 ships, China notably has the largest navy in the world and has now set out to achieve its priority weapons program.

A US Defense official told Reuters that the China's new first-in-class nuclear submarine -- a Zhou-class vessel -- sank when it was placed alongside a pier.

Though experts have said it is likely that there were casualities or that nuclear fuel was onboard the submarine, it is not known officially whether either of this is true, Wall Street Journal, which first reported the incident, said.

The submarine was eventually rescued but it is actually believed that it would be several months before it can be deployed back into the sea.

Reportedly, US officials have said that there is indication whether China checked the water or the nearby regions for any radiation. China's armed forces, the People's Liberation Army (PLA), and the government have notacknowledged the incident.

Thomas Shugart, a senior fellow at the Centre for a New American Security and a former US submarine officer, spotted unusual activity of cranes floating on the Wuchang shipyard on satellite images -- on social media -- and that is when the first indication of some sort of an incident came to be, WSJ reported.

Though Shugart suggested that an accident involving a submarine might have taken place, little did he know that it was nuclear-powered.

"Can you imagine a US nuclear submarine sinking in San Diego and the government hushes it up and doesn't tell anybody about it? I mean, holy cow!" he said.

The US official had also told Reuters that the silence over the incident raised severe questions over the accountability and competence of the Chinese military.

"In addition to the obvious questions about training standards and equipment quality, the incident raises deeper questions about the PLA's internal accountability and oversight of China's defense industry -- which has been plagued by corruption," the official said, adding that PLA Navy's attempt at concealing the sinking is "not surprising".

Meanwhile, a spokesperson of the Chinese Embassy in Washington said that they were not familiar with the incident and had no information to provide.

Beijing has placed the modernisation of the PLA Navy and its submarine as one of its top priorities given its bid to build a maritime military parity with the United States. As per Pentagon's latest report on Chinese military power report, Beijing had six nuclear-powered ballistic missile submarines, six nuclear-powered attack submarines and 48 diesel-powered attack submarines.

China's submarine force is likely to grow to 65 by 2025 and 80 by 2035, the US Defense Department was cited as saying.

The US Navy's entire submarine fleet is nuclear-powered. It has 53 fast-attack submarines, 14 ballistic-missile submarines and four guided-missile submarines.

A Pentagon official said that China's step towards developing new submarines along with its navy fleet is to counteract the United States act of coming to Taiwan's aid and establish "maritime superiority".

A senior research fellow at the Heritage Foundation thinktank told WSJ777pub, "The sinking of a new nuclear sub that was produced at a new yard will slow China's plans to grow its nuclear submarine fleet. This is significant."

• China
• Nuclear
• 777pub
• Lost
• Submarine