The US has announced it is moving an advanced missile defence system to the Pacific island of Guam as North Korea steps up its warlike rhetoric.
A statement from Pyongyang said its military had been given final approval to launch a nuclear strike.
North Korea has threatened to target the US and South Korea in recent weeks.
Its statement came amid warnings from US Defence Secretary Chuck Hagel that North Korea is a "real and clear danger" to the US and its allies.
And in a further development on Thursday, Pyongyang again blocked South Korean access to the Kaesong joint industrial zone 10km (six miles) inside North Korea. It was the second day the North had blocked the border crossing.
Another statement from the North threatened to close the complex down "if the South's puppet conservative group and its media continue bad-mouthing [us]".
The US Department of Defense said on Wednesday it would deploy the ballistic Terminal High Altitude Area Defense System (Thaad) in the coming weeks.
The Thaad system includes a truck-mounted launcher, interceptor missiles, and AN/TPY-2 tracking radar, together with an integrated fire control system.
The Pentagon said the missile system would be moved to Guam, a US territory with a significant US military presence, as a "precautionary move to strengthen our regional defence posture against the North Korean regional ballistic missile threat".
"The United States remains vigilant in the face of North Korean provocations and stands ready to defend US territory, our allies, and our national interests," the Pentagon added.
The US had already planned to send a Thaad system to Guam, but not under these circumstances, analysts say.
Riki Ellison, chairman of the Missile Defense Advocacy Group, told the BBC that the Pentagon's decision to speed up the transfer underlines how seriously Washington is taking Pyongyang's threats, but it should be viewed as a purely defensive move, not an escalation.
He said the Thaad system would only protect Guam and the surrounding area, and would not cover South Korea or Japan.
On Wednesday, a statement attributed to the North Korean military said: "We formally inform the White House and Pentagon that the ever-escalating US hostile policy towards the DPRK [North Korea] and its reckless nuclear threat will be smashed by the strong will of all the united service personnel and people and cutting-edge smaller, lighter and diversified nuclear strike means of the DPRK and that the merciless operation of its revolutionary armed forces in this regard has been finally examined and ratified."
It warned war could break out on the Korean peninsula as early as "today or tomorrow".
Caitlin Hayden, spokeswoman for the White House National Security Council, said the statement was "unhelpful and unconstructive".
"It is yet another offering in a long line of provocative statements that only serve to further isolate North Korea from the rest of the international community and undermine its goal of economic development," she said.
The BBC's Damian Grammaticas in Seoul says few observers believe North Korea has rockets or miniaturised weapons that could hit the US mainland.
Pyongyang could be seeking to pressure Washington to open fresh talks, hoping for a formal peace treaty, he adds.
In recent weeks, North Korea has mentioned military bases in Guam and the US state of Hawaii as possible targets.
"As they have ratcheted up her bellicose, dangerous rhetoric, and some of the actions they've taken over the last few weeks present a real and clear danger," said Mr Hagel, in his first major speech since taking up his post.
He added that Pyongyang had also threatened the interests of South Korea and Japan.
The North has apparently been angered by UN sanctions imposed after a recent nuclear test. Pyongyang escalated its rhetoric amid the current round of US-South Korea military drills.
The US has recently made a series of high-profile flights of stealth fighters and nuclear-capable B-52 bombers over South Korea.
Officials have also confirmed that the USS John McCain, a destroyer capable of intercepting missiles, has been positioned off the Korean peninsula.
A second destroyer, the USS Decatur, has been sent to the region.
China, the North's only powerful ally, said it had despatched officials on Tuesday to hold talks with ambassadors from North Korea, South Korea and the US.
On Wednesday North Korea closed the border crossing to the jointly-run Kaesong industrial park – the first time such action has been taken since 2009.
The complex is the last significant symbol of co-operation and is a key revenue source for North Korea.
The South Korean Unification Ministry on Thursday denied reports that North Korea had demanded South Korean workers quit Kaesong by 10 April.
It said the reports were based on a misunderstanding of a notice from the North.