WASHINGTON, Sept 28, (AP): Satellite imagery showed that China’s newest nuclear-powered attack submarine sank alongside a pier while under construction, a senior US defense official said. The sinking of China's first Zhou-class submarine represents a setback for Beijing as it continues to build out the world's largest navy.

Beijing has become increasingly assertive in pursuing its claim to virtually the entire South China Sea, which is crucial to international trade. Meanwhile, China faces longtime territorial disputes involving others in the region including Brunei, Malaysia, the Philippines, Taiwan and Vietnam. The United States has sought to strengthen ties to its allies in the region and regularly sails through those waters in operations it says maintains the freedom of navigation for vessels there, angering Beijing.

The submarine likely sank between May and June, when satellite images showed cranes that would be necessary to lift it off the bottom of the river, said the official, who spoke on condition of anonymity to provide details about the submarine loss. China has been building up its naval fleet at a breakneck pace, and the U.S. considers China’s rise one of its main future security concerns.

A Chinese Foreign Ministry spokesperson said Friday he was not familiar with the topic and did not provide any information when asked about it at a Beijing press conference. The US official said it was "not surprising” that China's navy would conceal it. The submarine's current status is unknown. The identification of the sunken nuclear submarine was first reported by The Wall Street Journal.

Thomas Shugart, a former US Navy submariner and an analyst at the Center for a New American Security, first noticed the incident involving the submarine in July, though it wasn’t publicly known at the time that it involved the new Zhou-class vessel. Satellite images from Planet Labs PBC analyzed by The Associated Press show what appears to be a submarine docked at the Shuangliu shipyard on the Yangtze River before the incident.