"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.
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.