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.
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 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.
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."
South Korean officials and Japanese media have reported that new intelligence shows North Korea is possibly preparing for a second nuclear bomb 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.
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.
Source: BBCGüncelleme Tarihi: 20 Eylül 2018, 18:16