About 50 North Korean submarines that disappeared from the radar amid heightened military tension are now returning to their bases, a military source said after the two Koreas reached a deal in the small hours of Tuesday.

"Some 50 submarines that had been away from their bases since Aug. 21 have shown signs of returning back to their bases," the military official said, adding that they had been moving in North Korea's inland sea.

The North had stationed special forces along the frontlines in order to strike at South Korean propaganda loudspeakers, and around 50 submarines left their heavily-guarded bases over the weekend as military tension heightened.

Experts said North Korea was intensifying saber rattling in order to gain the upper hand in marathon negotiations with South Korea that ended Tuesday morning. Experts said it made no sense from a strategic point of view to deploy 70 percent of North Korea's entire armada of submarines.

Moon Geun-shik at the Korea Defense and Security Forum, who used to command a submarine, said, "Even the U.S. does not deploy that many submarines at once. North Korea was probably intent on raising the pressure."