South Korea is on high alert after 50 North Korean subs deployed and subsequently disappeared from all radar scans.

There hasn’t been a submarine deployment this large since the conclusion of the Korean War in 1953, which is especially notable because North Korea has one of the biggest submarine fleets in the world — more than 70 subs in total.

“The North is adopting a two-faced stance with the talks going on,” a South Korea defense ministry spokesman said, according to AFP.

This latest deployment is more than 10 times the normal level. North Korea has also moved to double artillery units along the border, which flies in the face of an agreement reached Monday between the two sides to gradually lift the “semi-state of war.” This heated state of affairs came after a landmine injured two South Korean soldiers, an incident which Pyongyang said Tuesday that it regrets, Vice News reports.

Some observers were unsure about the sincerity of Pyongyang’s apology.

“I think the North had planned this out very specifically, planting these mines,” Philip Yun, a former negotiator with North Korea during the Clinton administration, told CNN. “They knew there was going to be a response from the South, therefore they could ratchet up or deescalate as they wished.”

Last week, hostilities included an exchange of artillery fire. As of Tuesday, South Korea has agreed to turn off loudspeakers blaring propaganda across the border. It’s been 11 years since the last time propaganda over speakers was needed.

South Korea is ramping up its surveillance efforts in response to submarine mobilization and preparing for potential attacks on either warships or commercial vessels. The last incident relating to a submarine occurred in 2010, when officials from South Korea accused the North of trying to take down a warship. The torpedo killed 46 South Koreans. Pyongyang denied any involvement.

U.S. military officials have begun reviewing plans to defend South Korea in the event that Pyongyang launches a much more serious assault.

Copyright 2015 The Daily Caller News Foundation