North Korean leader Kim Jong-Un told his soldiers to prepare for war, the Korean Central News Agency reported Saturday.
The order came one day after the US and South Korea conducted naval drills with 10 South Korean warships and a US missile destroyer, MSN News reports.
Kim was quoted at the opening of a new hall at the Victorious Fatherland Liberation War Museum in Pyongyang, and large scale military exercises were expected to follow the drills.
"The prevailing situation where a great war for national reunification is at hand requires all the KPA (Korean People's Army) units to become (elite) Guard Units fully prepared for war politically and ideologically, in military technique and materially," he said.
Kim added that he wants his military "to tear to pieces the Stars and Stripes". The threat was also directed at US allies.
An eight-week US-South Korea military exercise called Foal Eagle began on Monday, and involved 200,000 Korean soldiers and 3,700 American troops.
The training will take place on the ground, in the sea, and in the air, and will be followed by a week-long simulated drill called Key Resolve.
Both South Korea and the US have insisted that the exercises are defensive preparations, rather than offensive posturing.
North Korea had offered to cease nuclear testing if the drills were cancelled, but their offer was rejected by the White House.
On Thursday morning, the US ambassador to South Korea, Mark W Lippert, had his face and wrist slashed in Seoul by a man demanding Korean unification.
Lippert was preparing to give a lecture on the possibility of peace between North and South Korea when he was attacked outside a performing arts centre.
He reported that he was "doing well and in great spirits" after receiving 80 stitches in his face. Chung Nam-sik of the Severance Hospital said that the ambassador also sustained nerve damage in his left hand.
North Korea praised the attack, calling the slashing "a deserved punishment on war maniac US" and "knife slashes of justice."
