The North Korean military says it has ratified a "merciless" attack against the United States, potentially involving a "cutting-edge" nuclear strike.
"The moment of explosion is approaching fast," the army said in a statement on state news agency KCNA.
War could break out "today or tomorrow", the statement said, quoting a spokesman for the General Staff of the Korean People's Army.
"The merciless operation of (our) revolutionary armed forces in this regard has been finally examined and ratified.
"The US had better ponder over the prevailing grave situation."
The North's Committee for Peaceful Reunification of Korea was later quoted by KCNA as threatening to withdraw its 53,000 workers from the joint industrial zone it shares with the South.
Pyongyang informed Seoul on Wednesday that it was stopping the daily movement of South Koreans to the Kaesong complex, the last real surviving point of contact between the two countries.
And the committee said: "If the South Korean puppets and conservative news media keep badmouthing (us), we will order all our workers to pull out from Kaesong."
North Korea's latest pronouncements came as Washington scrambled to reinforce its Pacific defences, preparing to move an advanced missile defence system to the island of Guam.
The land-based weapon, which is primed to shoot down short and medium-range missiles, will be sent to the US territory to defend its bases there.
The Pentagon has already sent bombers, stealth aircraft and ships.
South Korea's Yonhap news agency said the North had moved a mid-range Musudan missile to its east coast.
The missile is believed to have a range of 1,875 miles (3,000km) or more, which would put all of South Korea and Japan in range and possibly also the US territory of Guam in the Pacific Ocean.
North Korea is not believed to have tested these missiles, according to most independent experts.
Tensions have been soaring on the Korean peninsula since the North launched a long-range rocket in December and conducted its third nuclear test in February.
North Korea has threatened missile and nuclear strikes against the US and South Korea in response to UN sanctions and joint military drills.
European diplomatic sources speaking to Sky News from the North Korean capital have said there is nothing there to suggest war is imminent: no sign of conscripts being signed up or unusual troop movements.
US Defence Secretary Chuck Hagel said on Wednesday that North Korea's "bellicose, dangerous rhetoric" posed a "real and clear danger" to America and its allies South Korea and Japan.
"They have nuclear capacity now, they have missile delivery capacity now," he said.
"We take those threats seriously, we have to take those threats seriously.
"I hope the North will ratchet its very dangerous rhetoric down."
The UK Government said it was not warning of an immediate risk to British citizens travelling to or living in South Korea.
In a statement to Sky News, the UK Embassy in Seoul said: "We have noted North Korea’s most recent statement, we are monitoring the situation and are in close contact with allies.
"We have been clear to North Korea that its long-term interests will not be served by threatening the international community and increasing regional tensions.
"We have updated our Travel Advice, advising British nationals in Korea and those travelling here to follow the advice of local authorities and subscribe to our travel advice, Twitter feed and Facebook page. We currently assess there is no immediate risk to British nationals in or travelling to Korea."
The tensions surrounding Kaesong - established in 2004 and a crucial source of hard currency for North Korea - carry enormous significance.
Neither of the Koreas has allowed previous crises to significantly affect the complex, which is the only surviving example of inter-Korean cooperation and seen as a bellwether for stability on the Korean peninsula.
China, the North's sole major ally, appealed for "calm" from all sides, repeating Beijing's oft-declared position.
Meanwhile, North Korea's media has become the latest victim of hacking group Anonymous, who have targeted the North's newswire service Uriminzokkiri.
The hackers broke through the defences of the newswire's Youtube, Twitter and Flickr sites to paste a series of spoof pictures featuring the North Korean flag and Kim Jong-Un, deformed into a pig.
Uriminzokkiri's twitter picture was replaced with a pair of Tango dancers wearing masks from the graphic novel V for Vendetta, used by Anonymous as their logo, next to a slogan "Tango Down".
Anonymous also issued a statement condemning the country's plans to develop nuclear weapons.