Kim-Jong-un graves sailors
North Korean dictator Kim-Jong-un visits the graves of sailors who died in a naval incident (KCNA)

Dozens of North Korean sailors died when at least one warship sank in an unexplained naval accident, it has emerged.

A submarine chaser sank off the Korean peninsula's east coast, KCNA, the North's official Korean Central News Agency, reported.

"Submarine chaser No 233 fell while performing combat duties," KCNA said.

The report did not say how many sailors died but added that measures to recover all the bodies were directed by the young dictator Kim Jong-un.

No further details were provided. The secretive dictatorship rarely releases information about military mishaps.

KCNA's report was accompanied by photos of Kim visiting a military cemetery where he paid tribute at a freshly built memorial to the dead. The images of at least 20 sailors encircled the memorial.

"[Kim] visited the cemetery of fallen fighters of KPA Navy Unit 790 who met heroic deaths while performing their combat duties," KCNA said.

"Kim Jong-un laid a flower and paid silent tribute to the memory of the fallen fighters."

A close-up shot showed the date of one death as 13 October, 2013.

South Korean media said not one but two warships sank off the naval port city of Wonsan during a military drill.

South Korea's conservative Choson Ilbo newspaper quoted an anonymous military source as identifying the two vessels as a Hainan-class 375-tonne submarine chaser and a 100-200-tonne patrol boat.

"The Hainan-class submarine chaser probably sank because it was old. It was built in China in the 1960s and the North bought it in the mid-70s," the source said. North Korean salvage crews were seen at the site of the accideent.

It was reported to have happened only one day after the US, South Korea and Japan held a jointed naval drill off the southern coast of the Korean peninsula.

"The US should know that our army is ready to confidently confront whatever turbulences and perilous provocations with powerful military forces," Pyongyang then commented.

Chinese news website Want China Times speculated that the vessels sank in a confrontation with the South Korean navy but that has not been confirmed.