The ever-ratcheting war of words between the two sides reached new heights after Pyongyang scrapped the agreement and then followed it up by ignoring Seoul's calls to a military hotline the two sides set up in 2004 to ease tensions.
'An act of war'
North Korea said its decision was a direct response to the U.N. Security Council, which passed tougher sanctions against the country after it carried out a nuclear test last month.
The sanctions, North Korea's ruling Workers' Party said, are "a declaration of war and an act of war."
The United States followed the U.N.'s sanctions with its own Monday after North Korea scrapped the armistice agreement.
The new U.S. sanctions target North Korea's Foreign Trade Bank for its role in supporting the country's weapons of mass destruction program, the Treasury Department said Monday.
The sanctions effectively cut the North's primary foreign exchange bank off from the U.S. financial system.
'Break the waists of ... enemies'
Pyongyang is also furious at joint military drills taking place between the South and the United States. The annual training exercises are scheduled to last two months.
Leader Kim Jong Un lambasted the drills, taking place near the Korean peninsula.