"The moment of explosion is approaching fast," the North Korean military said, warning that war could break out "today or tomorrow".
Pyongyang's latest pronouncement came as Washington scrambled to reinforce its Pacific missile defences, preparing to send ground-based interceptors to Guam and dispatching two Aegis class destroyers to the region.
Tension was also high on the North's heavily-fortified border with South Korea, after Kim Jong-Un's isolated regime barred South Koreans from entering a Seoul-funded joint industrial park on its side of the frontier.
In a statement published by the state KCNA news agency, the Korean People's Army general staff warned Washington that US threats would be "smashed by... cutting-edge smaller, lighter and diversified nuclear strike means".
"The merciless operation of our revolutionary armed forces in this regard has been finally examined and ratified," the statement said.
SEE: AFP: N. Korea approves nuclear strike on United States
More at Changing Worldviews
Sign Up for my Free Email Newsletter
Follow me on Facebook
Follow me on Twitter
Get Show on iTunes
Listen to my Radio Broadcasts on this subject here Broadcast page