"From this moment, the Supreme Command of the Korean People's Army will be putting in combat duty posture No. 1 all field artillery units including long-range artillery strategic rocket units that will target all enemy objects in U.S. invasionary bases on its mainland, Hawaii and Guam," North Korea said in a statement released by the Korean Central News Agency.
The announcement came as South Koreans marked the third anniversary of the sinking of a warship in which 46 South Korean sailors died. Seoul says the ship was hit by a North Korean torpedo, but the North denies involvement.
On Friday, the U.S. and South Korea signed a military pact providing for a joint response to even low-level provocation from the North.
Seoul's Defense Ministry said it hasn't seen any suspicious North Korean military activity, but they are analyzing the North's warning. North Korea has made similar statements in the past, but this was the first time the term "No. 1" was included.
Experts believe a direct North Korean attack is extremely unlikely, especially during joint U.S.-South Korean military drills that end April 30, but some worry about a provocation after the training wraps up.
North Korea has vowed to launch a nuclear strike against the U.S. and repeated its nearly two-decade-old threat to reduce Seoul to a "sea of fire." But experts say they have seen no proof that North Korea has mastered the technology needed to build a warhead small enough to mount on a missile.
ABC News and The Associated Press contributed to this report.