Satellite imagery showed that China‘s newest nuclear-powered attack submarine sank alongside a pier while under construction, a senior US defence 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 Phillippines, Taiwan and Vietnam.