North Korean sailors who died last month when their warship sank have been posthumously awarded the title of "patriotic martyr", the country's state media said Wednesday.

It was the second time in recent days that the official media has referred to the sinking -- an unusual admission of a military setback by the tightly controlled and secretive state.

The sailors who died "heroic deaths while performing their combat duties" have been given state commendations, martyr's certificates and membership cards of the ruling Workers' Party of Korea, the North's official news agency said.

"Commanding officers and sailors of submarine chaser No. 233 fell while performing their combat duties in mid-October last in defence of their post to the last moments of their lives," it said, offering no details of the incident.

The agency praised leader Kim Jong-Un for building a cemetery to "glorify forever their patriotic soul and revolutionary spirit".

"He personally visited the cemetery and became the owner of the cemetery, instructing the whole army to learn from their feats and noble spiritual world," it said.

An analyst said it was unusual for the North to disclose such a sensitive incident to the public.

"Kim is apparently using the tragic incident for his campaign to win loyalty from ordinary citizens and soldiers," said Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul.

The North's news agency did not say how many sailors were killed. But photographs of gravestones in Saturday's website edition of the ruling party newspaper Rodong Sinmun suggested about 15-20 may have died.

South Korea's Chosun Ilbo newspaper said Monday that two North Korean warships sank last month during an exercise off the eastern port of Wonsan, killing scores of sailors.

Quoting a military source, it said the ships were a Hainan-class 375-ton submarine chaser and a 100 to 200-ton patrol boat.

"The Hainan-class submarine chaser probably sank because it's old. It was built in China in the 1960s and the North bought it in the mid-70s," the source was quoted as saying.

While the North's military totals more than one million personnel, much of its equipment is ageing.