North Korea has threatened unspecified attacks on the US in an escalation of a war of words following the Sony Pictures cyber-attacks.
In a fiery statement, the North warned of strikes against the White House, Pentagon and "the whole US mainland".
North Korea denies US claims it is behind cyber-attacks linked to a film that features the fictional killing of its leader Kim Jong-un.
North Korea has a long history of issuing threats against the US.
The latest statement comes days after the US formally accused the North of orchestrating a massive cyber-attack on Sony Pictures.
"The army and people of the DPRK [North Korea] are fully ready to stand in confrontation with the US in all war spaces including cyber warfare space," a long statement carried by the official Korean Central News Agency said.
Analysis by Stephen Evans, BBC Korea correspondent
North Korea frequently uses fierce rhetoric against both South Korea and the United States so there's no great step-up in fierceness. And it is for domestic consumption as well as for outsiders.
The statement has weight because it comes from the most powerful body in North Korea, the National Defence Commission, which is chaired by Kim Jong-un.
It has two arguments - essentially "we didn't do it" and "whoever did do it was right".
The statement goes into some detail about the FBI argument that there were signs in the computer code that North Korea was behind the Sony attack. It said such lines of code are commonplace and do not prove any North Korean involvement.
It accused US President Barack Obama of "recklessly making the rumour" that North Korea was behind the Sony attack.
Separately, North Korea said it would not attend Monday evening's UN Security Council meeting discussing the country's human rights record.
Last week, the UN General Assembly passed a resolution calling on the Security Council to refer North Korea to the International Criminal Court to face charges of crimes against humanity.
A UN report released in February revealed ordinary North Koreans faced "unspeakable atrocities", including "deliberate starvation, forced labour, executions, torture" and political repression.
China, North Korea's main international ally, is expected to block any move against the country.
Sony details leaked
The Sony hack resulted in unreleased films and the script for the next James Bond movie being leaked online.
Details of corporate finances and private emails between producers and Hollywood figures were also released.
The eventual fallout from the attack saw Sony cancel the Christmas release of a comedy called The Interview, a film depicting the assassination of the North Korean leader.
That decision followed threats made by a group that hacked into Sony's servers and leaked sensitive information and emails.
The North has denied being behind the attacks, and offered to hold a joint inquiry with the US.
But the US turned down the offer, and President Obama said it was considering putting the North back on its list of terrorism sponsors, a move that further angered Pyongyang.
North Korea had been on the US list of state sponsors of terrorism for two decades until the White House removed it in 2008, as part of now-stalled negotiations relating to Pyongyang's nuclear programme.
In an interview with CNN on Sunday, Mr Obama promised to respond "proportionately" to the cyber-attack.
"I'll wait to review what the findings are," he said, adding that he did not think the attack "was an act of war".
The US has reportedly also asked China to curb cyber-attacks by North Korea.
China is seen as the nation with the most influence over Pyongyang.
Chinese Foreign Minister Wang Yi held a telephone conversation with his US counterpart John Kerry on Sunday in which they discussed the Sony row.
Mr Wang said China was "against all forms of cyber-attacks and cyber-terrorism" but did not refer directly to North Korea.
Correspondents say the issue of hacking is a sensitive one in Sino-US relations, with the two sides frequently trading accusations of cyber-espionage.
The Interview saga
The Interview features James Franco and Seth Rogen as two journalists granted an audience with Mr Kim. The CIA then enlists the pair to assassinate him.
Sony says it made the decision to cancel its release after most US cinemas chose not to screen the film, following terrorism threats.
- 22 November: Sony computer systems hacked, exposing embarrassing emails and personal details about stars
- 7 December: North Korea denies accusations that it is behind the cyber-attack, but praises it as a "righteous deed"
- 16 December: "Guardians of Peace" hacker group threatens 9/11-type attack on cinemas showing film; New York premiere cancelled
- 17 December: Leading US cinema groups say they will not screen film; Sony cancels Christmas-day release
- 19 December: FBI concludes North Korea orchestrated hack; President Obama calls Sony cancellation "a mistake"
- 20 December: North Korea proposes joint inquiry with US into hacks, rejected by the US