U.N. spokesman Martin Nesirky said the armistice is still in force because the armistice agreement had been adopted by the U.N. General Assembly, and neither North Korea nor South Korea could dissolve it unilaterally.
"We are certainly concerned by North Korea's bellicose rhetoric," White House spokesman Jay Carney told reporters Monday. "The threats that they have been making follow a pattern designed to raise tension and intimidate others. The DPRK will achieve nothing by threats or provocation, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in northeast Asia."
The move followed days of increased tensions over its latest nuclear test. North Korea was enraged with South Korea's military drills with the U.S. and with last week's U.N. sanctions.
North Korea also shut down a Red Cross hotline that North and South Korea used for general communication and to discuss aid shipments family reunions.
This isn't the first time North Korea threatened to nullify the armistice. It made the same threat several times before, even sending armed troops into a border village in 1996. The troops later withdrew.
The U.S. leveled new sanctions Monday against North Korea after it threatened to attack Washington with a nuclear weapon last week. Most experts agree that North Korea is incapable of doing so.
The Treasury Department designated North Korea's Foreign Trade Bank as a WMD proliferator and imposed similar punitive measures against Paek Se-Bong, the chairman of North Korea's Second Economic Committee, which oversees production of North Korea's ballistic missiles. The designation freezes any assets in the U.S. and bans Americans from doing business with the bank or Paek.
And the State Department placed similar restrictions on three other senior officials: Pak To-Chun, chief of North Korea's weapons production and arms exports department; Chu Kyu-Chang, a munitions industry director; and O Kuk-Ryol, who controls the North's nuclear research and development organization.
The Associated Press contributed to this report.