North Korea has put its missile units on standby to attack US military bases in South Korea and the Pacific.
The threat came after the US flew two nuclear-capable stealth bombers over the Korean peninsula.
North Korean leader Kim Jong-un signed off on the order at a meeting of top generals and "judged the time has come to settle accounts with the US imperialists in view of the prevailing situation", the official KCNA news agency said.
South Korea's Defence Ministry Spokesman Kim Min-seok also confirmed the meeting.
He said: "There was a report that at dawn today that North Korea's Kim Jong-un called an emergency military meeting and ordered missile units to standby.
"We see this as a continuing measure, after its army's supreme commander's announcement on 26 March that its army will take combat duty posture No 1."
The North has an arsenal of Soviet-era short-range Scud missiles that can hit South Korea and have been proven.
However, its longer-range Nodong and Musudan missiles that could in theory hit US Pacific bases are untested.
Mr Kim said that South Korea continues to monitor North Korean activities.
"This is seen as an extra measure in regards to its missiles. South Korean and US intelligence personnel are closely monitoring North Korea's readiness with its short, middle and long-range missiles such as Scud missile, Rodong missile and Musudan missile," he said.
The US yesterday flew two radar-evading B-2 Spirit bombers on practice runs over South Korea, responding to a series of North Korean threats.
They flew from the US and back in what appeared to be the first exercise of its kind, designed to show its ability to conduct long-range, precision strikes "quickly and at will", the US military said.
South Korea's Yonhap news agency reported there had been additional troop and vehicle movements at the North's mid and long-range missile sites, indicating they may be ready to fire.
It was impossible to verify the report, which did not specify a time frame, although South Korea's Defence Ministry said that it was watching shorter-range Scud missile sites, as well as Nodong and Musudan missile batteries.
Russia said that heightened military activity near North Korea was slipping into a "vicious cycle" that could get out of control.
Foreign Ministry Sergei Lavrov suggested that North Korea should cool down, calling on "all sides not to flex their military muscle" and avoid the danger of a belligerent response.
Russia supported new UN Security Council sanctions against North Korea in early March, but has criticised actions taken outside the council, including US and South Korean military drills.