North Korea says it has put its long-range missiles on standby for possible strikes on the United States, and its bases in South Korea and the Pacific.
The North Korean leader says he gave the order in response to Thursday's flight over South Korea of two United States nuclear-capable, stealth bombers.
State news agency KCNA says Kim Jong-un signed off on the order at a late-night meeting of top generals.
Kim Jong-un "finally signed the plan on technical preparations of strategic rockets of the KPA, ordering them to be stand-by for fire so that they may strike any time", the KCNA report said.
The time had come to "settle accounts" with the US, KCNA quoted him as saying, with the B-2 flights an "ultimatum".
Pyongyang has been angered by fresh UN sanctions and annual US-South Korea military drills, the BBC reports.
China, North Korea's biggest trading partner, has called on all sides to ease tensions.