North Korea has said its strategic rocket and long-range artillery units have been ordered to enter combat posture targeting US military bases on Guam, Hawaii and the mainland United States.
The announcement came after US bombers flew more sorties seen as threatening North Korea.
The order, issued in a statement from North Korea's military "supreme command", is the latest threat issued since the start of joint military drills by US and South Korean forces early this month.
South Korea's defence 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 US invasionary bases on its mainland, Hawaii and Guam," the North's KCNA news agency said.
The North has 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.
However, the US military's bases in the Pacific area are in range of its medium-range missiles.
South Korea's defence ministry said it had detected no signs of unusual activity by the North's military but will monitor the situation.
The South and the US 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 US military, accusing the US of war preparations by using B-52 bombers which have flown over the Korean peninsula as part of the drills.