South Korea's military said Sunday that it is keeping close tabs on the search for a North Korean midget submarine that went missing early last week.

The 70-ton Yugo class submersibles used by Pyongyang are usually used to infiltrate enemy lines in espionage operations. A U.S. cable news service first reported the submarine going missing, citing an unnamed intelligence source.

"It is believed the sub went missing while it was on a mission of some sorts," a military insider said. He speculated that the submarine may have lost power causing it to drift or sink.

The South Korean and U.S. military said they have observed efforts underway by the North Koreans to find the missing vessel.

"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, who declined to be identified, said.

The Yugo class boats are the same type as one that was caught in a fishing net in the East Sea in June 1998. These boats, in their infiltration role, can carry six on board, besides the two-person crew.

They are 21 meters-long, 2.75 meters wide and propelled by a diesel-electric system. They possess two 533-millimeter torpedo tubes.

The North is thought to have some 70 submarines in its inventory with the largest being the Soviet-era Romeo-class boats and the smallest being the Yugos. Besides the Yugo, the country's is known to have some 40 Sang-O class submarines with a displacement of 325 tons. One such vessel was found stranded in September 1996 near Gangneung, 237 kilometers east of Seoul.

It is widely believed that it was a Sang-O class sub that attacked and sank a South Korean corvette in March of 2010 that left 46 sailors dead. (Yonhap)