A US official has reportedly revealed that North Korea lost contact with one of its submarines that was operating off the North Korean coast last week.
Details about the class of the submarine are not known but the boat is 'presumed to have sunk' after it suffered a failure during an exercise, media sources reported.
The North Korean Navy's search operations to trace the missing submarine are being observed by US spy satellites, aircraft and ships, according to media sources.
A US official was quoted by USNI News as saying: "The North Koreans have not made an attempt to indicate there is something wrong or that they require help or some type of assistance."
It is believed that the submarine could be near one of North Korea's two submarine bases on the Sea of Japan.
The North Korean Military's fleet is comprised of more than 70 submarines, ranging from midget boats to larger boats.
The news comes at a time when the US Navy is participating in a biennial exercise, Ssang Yong 16 (SY16), in and around Pohang, Republic of Korea (ROK), to display their ability to conduct amphibious landing operations.
Nearly 9,200 marines, 3,100 US Navy sailors, 4,500 ROK Marine Corps (ROKMC), and 3,000 ROK Navy (ROKN) members are participating in SY16.