North Korea says UN sanctions imposed after its nuclear bomb test are a declaration of war, state media says.
Pyongyang announced it had carried out a nuclear test on 9 October
Pyongyang also warned of "merciless" blows against any country infringing on its sovereignty, the official KCNA news agency reported.
There are reports of new activity at last week's test site, and South Korea and Japan say they have intelligence of a possible second test.
The US says air samples have confirmed the 9 October blast was nuclear.
China has appealed to Pyongyang not to escalate tensions, urging its key ally to resolve the issue "through dialogue and consultation".
The UN Security Council unanimously voted on Saturday to impose sanctions, targeting Pyongyang's weapons and missile programmes as well as luxury goods.
In its first official statement since the vote, North Korea's foreign ministry said it "vehemently denounced" the resolution and called it a "product of US hostile policy" towards the country.
"The resolution cannot be construed otherwise than a declaration of a war" against the North, the statement from the foreign ministry said, echoing comments made by North Korea's ambassador to the UN on Saturday.
The statement repeated North Korea's previous claim that it wanted peace and a de-nuclearised Korean peninsula.
NEW UN SANCTIONS
Bans sale to, or export from, N Korea of military hardware
Bans sale or export of nuclear and missile related items
Bans sale of luxury goods
Freezes finances and bans travel of anyone involved in nuclear, missile programmes
Allows inspection of cargo to and from N Korea
Stresses new resolution needed for further action
But it warned "we will deliver merciless blows without hesitation to whoever tries to breach our sovereignty and right to survive under the excuse of carrying out the UN Security Council resolution."
US envoy Christopher Hill, who is in South Korea, warned North Korea not to carry out a second nuclear test.
"I think we would all regard a second test as a very belligerent answer on North Korea's part to the international community," he said.
Both South Korea and Japan have said they are aware of intelligence suggesting North Korea is possibly preparing for a second nuclear test.
A White House official said US spy satellites had detected suspicious movements near the 9 October blast site but these could not be confirmed as relating to a second test.
In the past, North Korea is thought to have staged activity at nuclear and missile sites to attract attention and step up diplomatic pressure, the BBC's Charles Scanlon in Seoul says.
N KOREA NUCLEAR PROGRAMME
Believed to have 'handful' of nuclear weapons
But not thought to have any small enough to put in a missile
Could try dropping from plane, though world watching closely
But analysts in Seoul say North Korea will need to conduct further tests in order to develop a credible nuclear deterrent, he adds.
US Secretary of State Condoleezza Rice has warned North Korea that a second nuclear test would be a provocative act that could only deepen its isolation.
She was speaking as she prepared for a tour of Asia to bolster the sanctions the UN has imposed on North Korea.
Her visit follows that of American envoy Christopher Hill, who has arrived in Seoul from Tokyo to continue talks on how to enforce the sanctions.
South Korea has publicly backed the sanctions outlined by the UN Security Council but it says it wants to continue joint economic projects with its neighbour despite the nuclear test.