"Seventy percent of North Korea's submarines left their bases, and their locations are not confirmed," a South Korean military source was quoted as saying by Yonhap.
North Korea is known to have a fleet of around 70 submarines.
Earlier, it was reported that the North has also doubled the number of its artillery forces along the border, near the demilitarized zone.
The news came just a day the conflicting neighbors held the first round of high-level talks to reduce tensions that escalated earlier this week. Earlier in the day, local media reported that Seoul and Pyongyang would continue the high-level talks later Sunday.
The tensions on the Korean peninsula escalated on Thursday, as North Korea reportedly fired shells at a South Korean military base across the demilitarized zone. Seoul was quick to retaliate with dozens of heavy artillery rounds.
