The warning came Monday from Pak Jae Gyong, vice-minister of the North Korean People's Armed Forces, in his address at a mass rally in Pyongyang, China's state-run Xinhua news agency reported, quoting official North Korean reports.
The rally, which the report said was attended by 100,000 people, had been called to condemn the latest U.N. Security Council resolution calling for stricter sanctions against North Korea for its nuclear and missile tests last month.
"Under the present situation where the Korean People's Army (KPA) is technically at war with the U.S. imperialists, and as the Armistice Agreement has lost its legal binding force, the KPA will promptly exercise the right to a pre-emptive strike to beat back the enemies' slightest provocation," Pak was quoted as saying.
The Armistice refers to the document ending the 1950-1953 Korean War.
North Korea already has vowed to develop its nuclear program and build more nuclear bombs in response to the U.N. resolution.
Pak threatened to deliver blows to the "vital parts of the U.S." and "wipe out all its imperialist aggressor troops no matter where they are in the world," the Xinhua report said.
Separately, Kim Ki Nam, secretary of North Korea's Central Committee of the Workers' Party, called the U.N. resolution "another grave provocation."
"This is, in essence, a wicked pressure offensive launched by the U.S. imperialists to disarm (North Korea), strangle its economy and undermine its ideology and system," he said.