North Korea says new UN sanctions that will restrict vital oil supplies are tantamount to a complete economic blockade.
It is the first official reaction from Pyongyang, a day after the UN Security Council unanimously passed new sanctions in response to North Korea's recent intercontinental ballistic missile test (ICBM).
In a statement carried by the official KCNA news agency, the North's foreign ministry said: "We define this 'sanctions resolution' rigged up by the US and its followers as a grave infringement upon the sovereignty of our Republic, as an act of war violating peace and stability in the Korean peninsula and the region and categorically reject the 'resolution'."
The new resolution is tantamount to a complete economic blockade of North Korea, the ministry added.
The US-drafted resolution seeks to ban nearly 90% of refined petroleum exports to the North by capping them at 500,000 barrels a year and demands the repatriation of North Koreans working abroad within 24 months.
It also caps crude oil supplies to the North at 4 million barrels a year and commits the UN Security Council to make further reductions if another nuclear test or ICBM launch was conducted.
North Korea's foreign ministry said those who voted for the sanctions would face Pyongyang's wrath.
"Those countries that raised their hands in favour of this 'sanctions resolution' shall be held completely responsible for all the consequences to be caused by the 'resolution' and we will make sure for ever and ever that they pay heavy price for what they have done."
It said its nuclear weapons "are a self-defensive deterrence not in contradiction of international law".
"We will further consolidate our self-defensive nuclear deterrence aimed at fundamentally eradicating the US nuclear threats, blackmail and hostile moves by establishing the practical balance of force with the US," it added.