"The merciless operation of (our) revolutionary armed forces in this regard has been finally examined and ratified," the statement said.
North Korea had threatened a "pre-emptive" nuclear strike against the United States a month ago, and last week the supreme army command ordered its strategic rocket units to combat status for strikes on the US mainland and US bases in Guam and Hawaii.
"The moment of explosion is approaching fast," Thursday's statement said, adding that a war could break out on the Korean peninsula "today or tomorrow".
"In view of this situation, the KPA General Staff in charge of all operations will take powerful practical military counteractions in succession," it said.
Despite a successful long-range rocket launch in December, most experts believe North Korea is years from developing a genuine inter-continental ballistic missile that could strike the mainland United States.
Hawaii and Guam would also be outside the range of its medium-range missiles, which would be capable, however, of striking US bases in South Korea and Japan.