China has honoured a submarine captain for averting an underwater "emergency" after his crew reportedly saved the vessel from sinking into waters thousands of metres deep, in a rare disclosure of a near-disaster.

Beijing has been increasing its naval might and reach in recent years, and it is an unusual admission for China's military, which President Xi Jinping regularly urges to strengthen its ability to "win battles".

Wang Hongli, commander of submarine 372, was praised by the Central Military Commission for "successfully dealing with a major sudden dangerous situation" during an underwater mission, the military's official People's Liberation Army Daily reported.

The crew "overcame various difficulties... and created a miracle in China's and the world's submarine history", said the report posted on the defence ministry website Wednesday, without specifying any details.

Submarine 372 is reportedly a diesel-electric Kilo-class vessel that went into service in 2006 after Beijing bought it from Moscow.

In April the PLA Daily reported that the submarine had been hit by a sudden change in water density while patrolling in an area of the sea "thousands of metres deep".

Pipes in the main engine room were damaged by the increased water pressure and the vessel began sinking rapidly after losing power, it said.

Wang and his crew reacted quickly, according to the report, shutting down nearly 100 valves and switches and operating dozens of controls -- all within three minutes -- before steering the submarine out of danger.

The crew risked their lives to "prove their loyalty", Wu Shengli, commander of the PLA Navy, was quoted as saying.

Military accidents and equipment malfunctions are rarely reported in China, where news about the armed forces is tightly controlled.