South Korea and the U.S. are taking part in annual military drills as North Korea followed through on its threat to nullify the ceasefire agreement that ended the Korean war.
Pyyongyang has launched a propaganda campaign against the drills, which involve an estimated 10,000 South Korean and 3,000 American troops.
The North Koreans also shut down a Red Cross hotline that helped North and South Koreans to communicate on aid shipments and reunions for separated families.
Pyongyang has piled threat on top of threat, including vows to launch a nuclear strike on the United States.
South Korean officials say U.S. troops there are meant to prevent North Korean aggression. Both South Korea and the U.S. say North Korea's recent talk and actions are more threatening than usual.
North Korea's official Korean Central News Agency quoted President Kim Jong Un saying the military is prepared for an "all out war."