A number of North Korean sailors were killed when a warship sank during "combat duties" last month, a state newspaper has reported in an unusual admission by the secretive state.

The North's ruling party newspaper Rodong Sinmun gave no figures for the number of dead. But photographs of gravestones in Saturday's website edition suggested about 15-20 may have died.

The paper showed solemn-faced leader Kim Jong-Un laying flowers at a cemetery specially created for victims of the incident, who "met heroic deaths while performing their combat duties".

The report gave no details of how the sailors on a ship identified as "submarine chaser no. 233" had died. It did not say where the cemetery was located.

After hearing of the incident, Kim ordered a search to retrieve all the bodies and gave detailed instructions on construction of the cemetery and gravestones, the paper said.

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.

North and South Korea have remained technically at war since the Korean conflict ended in an armistice in 1953.

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

Seoul accused Pyongyang of sending a submarine to sink a South Korean warship in March 2010 with the loss of 46 lives.