Days after North Korean leader Kim Yo Jong, supreme leader Kim Jong Un's sister, issued military threats to South Korea, sources and satellite images suggest that they weren't empty ones. An American reconnaissance flight which flew over the Korean peninsula reported the troop movement, confirming South Korean military sources.

In addition, new satellite images, which revealed a new submarine and secret naval base, also suggest that Pyongyang is beefing up military activities. As per South Korean military sources, North Korea stationed several troops at the demilitarized zone (DMZ) with its neighbor. The guard posts — around 150 in number — were empty following the inter-military Korean agreement. Such movements have also been spotted at the disputed Northern Limit Line which divides the two countries along the Yellow Sea.

The movement came after Pyongyang issued military threats to its neighbor following months of lull in the peace talks. On June 15, North Korea blew up a liaison office with South Korea in Kaesong. It was a result of the action from the North Korean defector activists who held rallies and sent food items to their former country using hot-air balloons.

"We are keeping an eye on related activities of the North Korean military. So far, no direct military action has been confirmed," a source with South Korea's joint chiefs of staff told media.

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Secret Naval Base

The troop movement was detected a day after a U.S.-based analyst Jacob Bogle shared Google Earth images that revealed secret underground naval base with dozens of subterranean maritime facilities and submarine at the Sinpo South shipyard. Such facilities can be used to launch attacks, refuel and repair ships. One such facility, which is near North Korea's dictator Kim Jong Un's residency, can be used as a shelter.

The submarine is a few yards out of the country's first ballistic missile submarine, Goare Class. In the same place, an upgraded Romeo-Mod missile boat is being built.

"In a war, North Korea plans to use submarines to harass South Korean shipping routes, the value of routes is over $1 trillion, and to enable the infiltration of special operations forces. Since their submarines can float in and out of hidden bases, their movements are harder to monitor and would pose a real threat to South Korea," Bogle, who studies North Korean movements using satellite images, told the media.

Although the exact scale of the base is hard to predict from commercial satellite images, Bogle believes that these bases have two access points. "It is likely that at least some of the larger bases contain more than a single, narrow tunnelway within its respective mountain," he said.

However, these bases have a fatal flaw -- "their entrance," he pointed out. Such underground bases are conventional and if the entrance is destroyed, the bases will be rendered useless as per Bogle.

Soldiers march during a grand military parade celebrating the 70th founding anniversary of the Korean
North Korean army during their 70th founding day parade Reuters

South Korean Military on Alert

The South Korean military has been on alert following the developments and demolition of liaison office. The Defense Ministry said on June 18 that if North Korea launches provocative military action, it will "pay the price."

Amidst the growing tensions between the two countries, South Korea's unification minister Kim Yeon Chul resigned taking full responsibility for the fallout. In his 14-month tenure, Kim never held a meeting with North Korean officials to cool the tensions as North Korea pulled out of the peacekeeping arrangements. His successor is yet to be announced.