The announcement, carried by KCNA news agency, follows days of strong rhetoric from Pyongyang.
It came as South Korea marked the third anniversary of the sinking of the Cheonan warship, which left 46 sailors dead.
South Korea said it had detected no signs of unusual activity in the North.
Tensions remain high on the Korean peninsula in the wake of North Korea's third nuclear test on 12 February. The test led to new UN sanctions which Pyongyang strongly opposes.
Joint US-South Korea annual military drills have further angered the communist nation. In recent weeks its habitually fiery rhetoric has escalated - it has threatened the US with "pre-emptive nuclear attacks", as well as strikes on US military bases in Japan.
"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 units and strategic rocket units that will target all enemy objects in US invasionary bases on its mainland, Hawaii and Guam," the KCNA statement said.
North Korea is not thought to have the technology to strike the US mainland with either a nuclear weapon or a ballistic missile, but it is capable of targeting US military bases in the region with its mid-range missiles.
North Korea's announcement comes as the country carries out large-scale military exercises, reports the BBC's Lucy Williamson in Seoul, and it is not clear whether the order applies to the army as a whole or just to specific units.'Path to survival'
The statement came hours after South Korean President Park Geun-hye called on North Korea to "change course" by abandoning nuclear weapons.
Ms Park called for change in Pyongyang as she honoured the Cheonan deadSpeaking at the national cemetery in Daejeon where the sailors who died when the warship sank are buried, Ms Park said North Korea continued to threaten the South's national security.
"For the North, the only path to survival lies in stopping provocations and threats, abandoning its nuclear weaponry and missiles, and becoming a responsible member of the international community," she said.
The Cheonan sank in 2010 near the disputed inter-Korean western maritime border. South Korea says a North Korean torpedo sank the ship but Pyongyang denies any involvement.
Months later, North Korea shelled a border island in the same area, killing four South Koreans, including two civilians. It said it was responding to military exercises in the South.
Last week, South Korea and the US signed a new military plan - conceived after the shelling - pledging joint action to counter limited attacks from North Korea.
