The North Korean army has warned Washington that its military has been cleared to wage an attack using "smaller, lighter and diversified nuclear" weapons.
"The moment of explosion is approaching fast," the military said, warning that war could break out "today or tomorrow".
The statement, carried by the Korean Central News Agency early on Thursday, said troops had been authorised to counter US aggression with "powerful practical military counteractions".
The warning, the latest in a series of escalating threats against the US and South Korea, came after the Pentagon said it would deploy a missile defence system to the US Pacific territory of Guam to strengthen the region's protections against a possible attack.
Despite the rhetoric, analysts say they do not expect a nuclear attack by North Korea, which knows the move could trigger a destructive, suicidal war that no one in the region wants.
Ban Ki-moon, the UN Secretary-General, said that he was "deeply concerned" over rising tensions on Thursday.
US Defence Secretary Chuck Hagel, however, labelled North Korea's recent rhetoric as a threat to the US and its Asia-Pacific allies.
"Some of the actions they've taken over the last few weeks present a real and clear danger," Hagel told an audience at the National Defence University in Washington on Wednesday.
He added that the US is doing all it can to defuse the situation.
Deployment of the Terminal High Altitude Area Defence System is the latest step the US has taken to bolster forces in the region in a far-reaching show of force aimed at countering the North Korean threat.
The Pentagon already has sent dispatched bombers, stealth fighters and ships.
The land-based missile defence system includes a truck-mounted launcher, tracking radar, interceptor missiles, and an integrated fire control system.
The Pentagon said the system will boost defences for American citizens in Guam, a US territory, and US forces stationed there.
North Korea has railed for weeks against joint US and South Korean military exercises taking place in South Korea and has expressed anger over tightened sanctions for a February nuclear test.
Following through on one threat Wednesday, North Korean border authorities refused to allow entry to South Koreans who manage jointly run factories in the North Korean city of Kaesong.