A North Korean People's Navy submarine has reportedly gone missing and is presumed to have sunk. The vessel was believed to be operating off the pariah state's coast.

"About week ago it went missing and the speculation is that it sank," an American official told the US Naval Institute's news website. "The North Koreans have not made an attempt to indicate there is something wrong or that they require help or some type of assistance."

The official did not give further details on the loss of the boat due to sensitivities on how the military was tracking the vessel, the site reported. The US is known to monitor the country, which has nuclear capabilities, closely.

The report emerged as North Korea threatened to retaliate against US and South Korean forces taking part in annual joint military drills, known as Foal Eagle or Ssangyong.

Accusing allies South Korea and the US of "becoming more reckless as the days go by" North Korea threatened "an ultra-precision blitzkrieg strike of the Korean style".

North Korea calls foul

Conducted every spring since 1997, this year's Foal Eagle war games involved 17,000 South Korean and US troops, including marine forces from both sides. Numerous amphibious assault vehicles are also deployed.

While the allies say they war games are routine and defensive, the North calls the drills a rehearsal for an invasion. Technically North and South Korea are all still at war, because the 1950-1953 Korean War ended in an armed truce instead of a peace agreement.

To protest the massive war games held near its borders, North Korea fired two short-range ballistic missiles into the sea on 10 March in defiance of United Nations Security Council resolutions.

The UN imposed tough sanctions on Pyongyang following a recent nuclear test and long-range missile launch. South Korea's foreign ministry said it would refer the matter to the Council's sanctions committee mandated to enforce the resolutions.