North Korea said it would "show off our army and people's stern reaction to safeguard our sovereignty and the highest dignity through military actions".
The Pentagon is prepared to "respond to any contingency" after North Korea again threatened to strike targets in Guam, Hawaii and the continental US.
North Korea's Supreme Command announced on Tuesday that it had elevated its artillery and strategic missile forces to "combat-ready posture", adding that it was prepared to strike South Korea, Japan and US territory.
North Korea said it would "show off our army and people's stern reaction to safeguard our sovereignty and the highest dignity through military actions".
The comments were condemned by the Pentagon as "bellicose rhetoric". "We are concerned by any threat raised by the North Koreans," said George Little, a spokesman. "We take everything they say and everything they do very seriously. They need to stop threatening peace - that doesn't help anyone."
He added: "North Korea will achieve nothing by threats or provocations which will only further isolate North Korea and undermine international efforts to achieve peace and stability in north-east Asia. We stand ready to respond to any contingency."
The threats come a day after Kim Jong-un was pictured visiting military units and watched exercises on the east coast involving troops storming ashore from hovercraft and shelling targets with artillery.
The statement, via the Korean Central News Agency (KCNA), said North Korea can no longer overlook Washington's nuclear and military threats.
The US and South Korea recently conducted military exercises involving a nuclear submarine and B52 bombers flying from airbases on Guam.
"The US nuclear war racket has gone beyond the danger line and entered the phase of an actual war, defying the repeated warnings from the army and people of the Democratic People's Republic of Korea," the statement said.
South Korea received another round of threats, which have become more frequent and more vitriolic since the United Nations Security Council voted unanimously to step up sanctions against Pyongyang for carrying out a nuclear test in February. The KCNA report warned that the government in Seoul "should be mindful that everything will be reduced to ashes and flames the moment the first attack is unleashed".
Despite the threats, North Korea does not have the capability to carry out its latest threat, according to experts.
James Hardy, the Asia Pacific Editor from IHS Jane's Defence Weekly, said: "From what we know of its existing inventory, North Korea has short and medium-range missiles that could complicate a situation on the Korean Peninsula - and perhaps reach Japan - but we have not seen any evidence that it has long-range missiles that could strike the continental US, Guam or Hawaii."
Pyongyang's latest threats were issued on the day that South Korea marked the third anniversary of the sinking of the corvette Cheonan in a torpedo attack blamed on the North.
Memorial ceremonies were held across the country for the 46 South Korean sailors killed when the warship exploded and sank in the Yellow Sea. North Korea maintains that it had nothing to do with the incident.
Hugo Swire, the foreign office minister, told The Daily Telegraph in Beijing that the regime in Pyongyang remains "very unpredictable" and the hope remains that Beijing might be able to intervene to rein in its neighbour.
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