North Korea has vowed to restart its uranium enrichment programme and "weaponise" all the plutonium in its possession.
The warning - from North Korea's Foreign Ministry - came hours after the UN Security Council approved tough new sanctions on the North to punish it for its latest nuclear test on May 25.
It also threatened to take "resolute military action" if the US or its allies try to impose any "blockade" on it.
It did not elaborate if the blockade refers to an attempt to stop its ships or impose sanctions.
North Korea also said it would not abandon its nuclear programmes, saying it was an inevitable decision to defend itself from what it says is a hostile US policy and its nuclear threat against the North.
North Korea describes its nuclear programme as a deterrent against possible US attacks. Washington says it has no intention of attacking and has expressed fear that North Korea is trying to sell its nuclear technology to other nations.
The UN resolution imposes new sanctions on the reclusive communist nation's weapons exports and financial dealings, and allows inspections of suspect cargo in ports and on the high seas.
The South Korean government said it "welcomes and supports the unanimous adoption of the resolution."
