North Korea has reportedly lost a submarine

SEOUL -- The U.S. and South Korean militaries are reportedly keeping close tabs on the search for a North Korean submarine believed to have sunk, according to South Korean media.

"It is believed the sub went missing while it was on a mission of some sort," South Korea's Yonhap news agency cited a military insider as saying.

The U.S. Naval Institute quoted an American official on Friday as saying the sub was presumed to have sunk.

The submarine was a 70-ton Yugo class sub, usually used by the North to infiltrate enemy lines in espionage operations, according to Yonhap.

According to a BBC report, the vessel had been operating off the North Korean coast for several days when it disappeared. The vessels can only carry about eight people.

"Because these subs are very old, they are susceptible to mechanical breakdowns and due to North Korea's economic situation, it is not likely that they have been well maintained," the military official told Yonhap.

The news agency said the South Korean and U.S. military -- currently conducting massive joint drills around the Korean Peninsula -- have been closely monitoring the North's efforts to find the missing vessel.