Story highlights North Korean military said it will "physically act" against THAAD

THAAD is an advanced defense system that can intercept ballistic missiles

Seoul (CNN) North Korea's military has threatened to "physically act" against an advanced anti-missile system deployed by the U.S. and South Korea, and make them "suffer from the nightmare extreme uneasiness and terror."

The harsh rhetoric by North Korea's People's Army (KPA) is the communist state's first comment on Terminal High Altitude Area Defense (THAAD) since the U.S. and South Korea announced Friday they are in the final stages of negotiation about where to deploy the missile system.

THAAD can shoot down short, medium and intermediate ballistic missiles at incredible speed and altitude, and has been used by the U.S. for years to protect its military units.

"We once again warn the enemies that it is the steadfast will of the KPA to make merciless retaliatory strikes to reduce South Korea to a sea in flames and debris once an order is issued," KCNA cited the Korean People's Army as saying.