WASHINGTON • A North Korean submarine is missing, reports said yesterday, as the reclusive state issued a fresh threat of retaliation against US and South Korean forces involved in joint military drills.

The unknown class of vessel had been reportedly operating off the North Korean coast earlier in the week when it disappeared.

A South Korean Defence Ministry official said Seoul was investigating the reports. Pentagon officials declined to comment on the matter.

The US military had been observing the submarine off the North's eastern coast, CNN said, citing three US officials familiar with the incident. American spy satellites, aircraft and ships have been watching as the North Korean navy searched for the missing sub, the report added.

The US is unsure if the missing vessel is adrift or whether it has sunk, CNN reported, but officials believe it suffered a failure during an exercise. The US Naval Institute (USNI) News said the submarine was presumed sunk.

The speculation is that it sank," an unidentified US official was quoted as telling the USNI News. "The North Koreans have not made an attempt to indicate there is something wrong or that they require help or some type of assistance."

North Korea's navy operates a fleet of some 70 submarines, most of them rusted diesel submarines that are capable of little more than coastal defence and limited offensive capabilities.

The submarine's disappearance comes as tensions were further heightened on the Korean Peninsula by a fresh threat from Pyongyang.

The official KCNA news agency, citing a statement from military chiefs, warned of a "pre-emptive retaliatory strike at the enemy groups" involved in the joint US-South Korean drill.

Pyongyang added it planned to respond to the drills with an "operation to liberate the whole of South Korea, including Seoul" with an "ultra-precision blitzkrieg".

Responding to the statement, South Korea's Defence Ministry urged Pyongyang to stop making threats or further provocations, according to the Yonhap news agency.

US and South Korean troops staged a big amphibious landing exercise yesterday, storming simulated North Korean beach defences. The landing and assault drills on South Korea's east coast were part of eight weeks of joint exercises between the allies, which the South has said are the largest ever.

About 55 US marine aircraft and 30 US and South Korean ships, including the USS Bonhomme Richard and USS Boxer, which carry AV-8B Harrier attack jets and V-22 Osprey aircraft, took part in the assault on beaches near Pohang city, the US navy said.

"They will penetrate notional enemy beach defences, establish a beach head, and rapidly transition forces and sustainment ashore," the US military based in South Korea said in a statement before the exercise.

The South Korean and US militaries have said they had notified the North of "the non-provocative nature" of the exercises involving about 17,000 American troops and more than 300,000 South Koreans.

AGENCE FRANCE-PRESSE, REUTERS