Two-thirds of North Korea’s submarine fleet was reportedly on the move and off of Seoul’s sonar this week despite an announcement by the two Koreas that they were ratcheting down the saber rattling that followed a land mine explosion in the demilitarized zone earlier this month.

More than 50 North Korean subs, believed to represent about 70 percent of Pyongyang’s fleet — were still unaccounted for Wednesday in a potentially ominous development that a spokesman for South Korea's Defense Ministry called "unprecedented." Seoul and the U.S., which maintains a strong presence in South Korea, responded by increasing military surveillance.

"The number is nearly 10 times the normal level … we take the situation very seriously," Kim Min-seok, the defense ministry spokesman, said Tuesday.

"No one knows whether the North will attack our warships or commercial vessels."

- Kim Min-seok, South Korea defense ministry spokesman

South Korean news agency Yonhap quoted a military official as saying the country was "mobilizing all our surveillance resources" to find the missing subs. Yonhap also reported that the submarines, which slipped away from their bases on Friday, likely had returned to naval bases in North Korea. But until they are accounted for, officials say their is concern on the seas surrounding the peninsula.

“We’ve said before the disappearance [of North Korean submarines] is a source of concern, and the fact is they are not easy to detect when they are submerged under water,” Kim said. "No one knows whether the North will attack our warships or commercial vessels," the defense ministry official said.

Pyongyang has also used amphibious landing craft to move special forces near the two nation's maritime border on the Yellow Sea, Yonhap reported Monday.

Tensions between the two nations flare up from time to time, often due to North Korean aggression. In 2010, North Korea’s navy was accused of torpedoing a South Korean warship in an attack that killed 46 people. Pyongyang denied responsibility.

Monday’s announcement that the two nations would dial down tensions came after several days of talks in the border village of Panmunjom, in which South Korea agreed to stop blasting propaganda from loudspeakers at the militarized border. Pyongyang said it regretted the land-mine blast earlier this month that injured two South Korean soldiers.

It also followed an exchange of artillery fire at the border last week, which South Korea said was started by the North.

The two countries have technically been at war since the 1950s, often coming to the apparent brink of all-out hostilities only to step back. Jonathan Pollack, a Korea expert at the Brookings Institute, said the weekend talks that appeared to have calmed the waters included a rare admission from North Korea that its land mines had detonated. Pollack said his sources say the submarines had headed back to port and were no longer accounted for, and that their temporary disappearance was part of the latest round of tensions.

"For its own reasons, North Korea built this up, and then for its own reasons ratcheted it down," he said. "I don't discount their threats, but they express them regularly, sometimes against the U.S. and often for things they don't have the capability to do."

The submarines initially left their ports at the height of the crisis, and the motive behind their deployment was not known, said Scott Snyder, of the Council on Foreign Relations.

"It is not clear whether this is a defensive or offensive move; thus it requires continued watchfulness," Snyder said.

On Monday, Pentagon spokesman Capt. Jeff Davis said there has been an increase in military operations by North Korea, but that it “has not been at a level that is high enough to cause alarm.”

South Korean President Park Geun-hye had ordered the propaganda blasted from loudspeakers at the border until the regime led by third-generation dictator Kim Jong Un took responsibility for the three land mines planted there. In a statement released by her office, she said: "We need a clear apology and measures to prevent a recurrence of these provocations and tense situations. 

"There will be no retreat in the face of North Korean threats," she added.

Pollack said North Korea's statement expressing "regret" over the land mine incident was as close to an apology as Pyongyang ever gives.