North Korea has ramped up rhetoric ahead of a UN vote on sanctions in response to its nuclear test.
Accusing the US of pushing to start a war, it vowed to exercise its right to launch a pre-emptive nuclear attack against its aggressors.
The Security Council meets later today to approve fresh sanctions against Pyongyang over the 12 February test.
Earlier this week, North Korea also threatened to scrap the 60-year truce which ended the Korean War.
“As long as the United States is willing to spark nuclear war our forces will exercise their right to a pre-emptive nuclear strike,” said North Korea’s foreign ministry, in a statement carried by the KCNA news agency, without giving further details.
The BBC’s Lucy Williamson in Seoul says most analysts believe Pyongyang is unlikely to start a war with the US, and may instead be trying to provoke a fresh stance from Washington ahead of the UN vote.
But the atmosphere on the Korean Peninsula is more tense than usual, our correspondent adds, after North Korea said that it would tear up the armistice agreement next week.
The two Koreas remain technically at war in the wake of the 1950-53 Korean War, which ended with an armistice, not a formal peace treaty.
North Korea’s military command said it would end that armistice on 11 March, threatening “surgical strikes” on its southern neighbour and the use of a “precision nuclear striking tool” in response to the sanctions and ongoing South Korea-US military drills.
South Korea warned earlier this week that it would respond to any provocation from its northern neighbour.
On Thursday, a spokesman for the South Korean Defence Ministry, Kim Min-seok, said Pyongyang had been observed carrying out “various drills involving the army, navy and air force,” and that further exercises were being prepared, the Yonhap news agency reports.
He said South Korea was taking “great interest in these activities,” giving the view that such training could be “extended into a provocation at any time”.
Officials also told Yonhap the North had imposed no-fly and no-sail zones off its east and west coasts, an indication that it could be planning to test-fire missiles.
Seoul is itself conducting military drills at the moment, in its annual joint manoeuvres with the US army, involving some 13,000 troops from both sides. The North views the drills as a provocation and as preparation for war.
The UN Security Council, meanwhile, will vote on the sanctions resolutions on North Korea at 10:00 EST (15:00 GMT).
The resolution, which is expected to pass, was proposed by China and the US, and will target North Korea’s diplomats, cash transfers and access to luxury goods.
It will also impose asset freezes and travel bans on three individuals and two corporations linked to North Korea’s military.
The sanctions have been described by Washington’s UN ambassador Susan Rice as “some of the toughest sanctions” the UN had ever imposed.
Meanwhile, Australia has put on hold plans to reopen a North Korea embassy in its capital, Canberra.
Patrick Low, a spokesman for the Australian foreign ministry, said there was still “some merit” in having a North Korean embassy in Australia, including enabling more direct communication on human rights issues.
But said the plans had been frozen “until further notice” while Australia worked with the UN Security Council on its response to North Korea’s nuclear test.
North Korea first opened an embassy in Australia in 2002, but closed it in 2008 for financial reasons.
The nuclear test, North Korea’s third, followed its apparently successful launch in December of a three-stage rocket – a move condemned by the UN as a banned test of missile technology.
Pyongyang claims its nuclear test involved a smaller and more powerful device – prompting concerns it could be moving closer to creating a warhead small enough to arm a missile.