More than 50 North Korean submarines — about 70 percent of the country's known fleet — have reportedly left their bases and disappeared from South Korea's military radar, a development that comes as the two Koreas announced on Monday that they plan to lift the "semi-state of war" prompted by a land mine explosion that injured two South Korean soldiers on August 4.

A spokesman for South Korea's Defense Ministry reportedly said the North's mass submarine deployment is "unprecedented," and that the US and Seoul have responded by increasing their military surveillance.

"The number is nearly 10 times the normal level… we take the situation very seriously," the Defense Ministry spokesman said, according to AFP.

South Korean news agency Yonhap quoted a military official as saying the country is "mobilizing all our surveillance resources" to find the missing subs. "No one knows whether the North will attack our warships or commercial vessels," the South Korean official reportedly said.

Seoul previously accused the North of torpedoing a South Korean warship in 2010 in an attack that killed 46 people. Pyongyang denied responsibility.

Yonhap also reported Monday that North Korea deployed 20 air-cushion landing crafts near the Yellow Sea border with the South. The amphibious landing craft reportedly left a North Korean naval base in Cholsan, North Pyongan Province.

As part of the agreement announced Monday afternoon after several days of marathon negotiations, South Korea agreed to stop blasting propaganda from loudspeakers pointed at the militarized border with the North. Pyongyang said they regretted the land mine blast earlier this month. Further details of the talks were not disclosed.

North Korea called the blaring propaganda an "act of war," and criticized a joint military exercise between the US and South Korea.

The announcement of a truce comes after the two Koreas exchanged live artillery fire between the border last Thursday and Friday, which South Korea said was started by the North. It was the first time that North Korea directly attacked South Korean land since 2010, when they shelled the island of Yeonpyeong, killing four people.

South Korea hadn't blared messages across the border in 11 years, but it's an old tactic the South has used on and off for decades to infuriate its northern neighbor. The messages criticized North Korean leader Kim Jong-un and the country's notoriously closed-off political system.

The two countries have technically been at war since the 1950s, with occasional skirmishes breaking the ceasefire in the meantime. The last time the two countries exchanged fire was last October, after North Korea fired on South Korean balloons that were dropping propaganda materials near the border.

This story has been updated since it was initially published to include details about the agreement reached between North and South Korea.

Top photo released by North Korea's state news agency __shows North Korean army and navy soldiers conducting a joint drill with amphibious landing crafts on an unidentified North Korean beach.

_Follow VICE News on Twitter: _@vicenews