The order, issued in a statement from the North's military "supreme command", marks the latest fiery rhetoric from Pyongyang since the start of joint military drills by U.S. and South Korean forces early this month.
South Korea's defense ministry said it saw no sign of imminent military action by North Korea.
"From this moment, the Supreme Command of the Korean People's Army will be putting into combat duty posture No. 1 all field artillery units, including long-range artillery units and strategic rocket units, that will target all enemy objects in U.S. invasionary bases on its mainland, Hawaii and Guam," the North's KCNA news agency said.
The North previously threatened nuclear attack on the United States and South Korea, although it is not believed to have the capability to hit the continental United States with an atomic weapon. But the U.S. military's bases in the Pacific area are in range of its medium-range missiles.
South Korea's defense ministry said it had detected no signs of unusual activity by the North's military but will monitor the situation. The South and the U.S. military are conducting drills until the end of April, which they have stressed are strictly defensive in nature.
The North has previously threatened to strike back at the U.S. military accusing Washington of war preparations by using B-52 bombers which have flown over the Korean peninsula as part of the drills.
North Korea has said it has abrogated an armistice that ended the 1950-53 Korean War and threatened a nuclear attack on the United States.
(Reporting by Jack Kim and Ju-min Park; Editing by Neil Fullick)