Pyongyang has recently carried out nuclear tests and rocket launches
North Korea said it will "weaponise" its plutonium stocks amid threats to take military action over United Nations sanctions, state media said.
Pyongyang has for the first time confirmed it is seeking to enrich uranium in efforts to develop nuclear weapons, it said.
North Korea would view any US-led attempts to "blockade" it as an "act of war", the Associated Press (AP) said.
It follows a toughening of UN sanctions against the communist state.
The warning from North Korea's foreign ministry was carried by Pyongyang's official Korean Central News Agency (KCNA) on Saturday.
On Friday, the United Nations Security Council voted unanimously to impose tougher sanctions on communist North Korea, after its nuclear test on 25 May.
The North also tested a rocket thought to be capable of carrying a warhead, though Pyongyang said it was designed to carry a satellite.
The UN sanctions include the inspection of North Korean ships, a wider ban on arms sales and other financial measures.
The US deputy ambassador at the UN, Rosemary DiCarlo, said the new vote was a strong and united response to North Korea's "unacceptable behaviour".
The North Korean foreign ministry statement said: "Firstly, all plutonium to be extracted will be weaponised. One third of used fuel rods have so far been reprocessed.
"Secondly, we will start uranium enrichment," the statement added.
The BBC's John Sudworth in Seoul said the admission that Pyongyang was trying to enrich uranium was worrying, as the processing could take place in a smaller reactor and was easier to hide.
Korea analyst Aidan Foster Carter told the BBC Pyongyang's process was "out of control" and that nothing seemed able to persuade North Korea to stop its nuclear ambitions - neither sanctions nor financial incentives.
He said the nuclear stand-off may be part of internal ructions as Pyongyang's leader Kim Jong-il decides which of his three sons will take over from him.
Handful of weapons
North Korea is thought to possess enough reprocessed plutonium for between six and eight nuclear weapons.
However, analysts say Pyongyang has not yet mastered the technology to make a nuclear warhead small enough to place on a missile.
Pyongyang has stated its nuclear weapons programme is purely a defensive measure to protect it against a US attack.
Washington has said it does not intend to attack the North, and is concerned Pyongyang's nuclear knowledge could be passed to other states.