North Korea has threatened the US with a pre-emptive nuclear strike, raising the level of rhetoric just before the UN Security Council approved new sanctions against the reclusive country.
The White House said the threats would only lead to Pyongyang's further international isolation and declared the US was "fully capable" of defending against any attack.
China's UN Ambassador Li Baodong said Beijing wanted to see "full implementation" of the new UN Security Council resolution that tightens financial restrictions on Pyongyang and cracks down on its attempts to ship and receive banned cargo.
North Korea has accused the US of using military drills in South Korea as a launch pad for a nuclear war and has scrapped the armistice with Washington that ended hostilities in the 1950-53 Korean War.
A North Korean general said on Tuesday that Pyongyang was scrapping the armistice.
But the two sides remain technically at war as the civil war did not end with a treaty.
North Korea threatens the US and its "puppet," South Korea, on an almost daily basis.
"Since the United States is about to ignite a nuclear war, we will be exercising our right to pre-emptive nuclear attack against the headquarters of the aggressor in order to protect our supreme interest," the North's foreign ministry spokesman said in a statement carried by the official KCNA news agency.
North Korea conducted a third nuclear test on 12 February in defiance of UN resolutions, and declared it had achieved progress in securing a functioning atomic arsenal.
It is widely believed that the North does not have the capacity for a nuclear strike against the mainland of the US.
With tensions high on the Korean peninsula, the UN Security Council voted unanimously to expand its sanctions on North Korea.
The new sanctions were agreed after three weeks of negotiations between the United States and China, which has a history of resisting tough measures against its ally and neighbour.
The resolution specifies some luxury items North Korea's elite is not allowed to import, such as yachts, racing cars, luxury automobiles and certain types of jewellery.
This is intended to close a loophole that had allowed countries to decide for themselves what constitutes a luxury good.
"These sanctions will bite and bite hard," said US Ambassador to the United Nations Susan Rice.
The export of luxury goods to North Korea has been prohibited since 2006, though diplomats and analysts said the enforcement of UN sanctions has been uneven.
UN Secretary-General Ban Ki-moon, a former South Korean foreign minister, welcomed the council's move, saying in a statement that the resolution "sent an unequivocal message to (North Korea) that the international community will not tolerate its pursuit of nuclear weapons."
The success of the new measures, council diplomats said, will depend to a large extent on the willingness of China to enforce them more strictly than it has in the past.
Pyongyang was hit with UN sanctions in retaliation for its 2006 and 2009 nuclear tests. Those measures were subsequently tightened and expanded after several rocket launches by the North.
In addition to the luxury goods ban, there is an arms embargo on North Korea, and it is forbidden from trading in nuclear and missile technology.