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 in 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. -AFP