The claim comes as some U.S. intelligence analysts are becoming increasingly concerned that the communist regime may have as many as six nuclear weapons instead of the one or two the CIA now estimates.
New atomic bombs for North Korea also could mean that the Stalinist regime might part with one bomb, either in a test or by selling it. A North Korean official, however, told reporters in New York his government would not export its nuclear capability.
"The (North) successfully finished the reprocessing of some 8,000 spent fuel rods," a spokesman of Pyongyang's Foreign Ministry said in a statement carried by the North's official news agency KCNA.
Accusing the United States of taking a "hostile policy" toward the North, the statement said North Korea "made a switchover in the use of plutonium churned out by reprocessing spent fuel rods in the direction (of) increasing its nuclear deterrent force."
When reprocessed with chemicals, the 8,000 rods can yield enough plutonium for North Korea to make five or six more nuclear weapons, according to experts.
North Korea has claimed before that it has completed reprocessing its pool of 8,000 spent rods, but Thursday's statement clarified for the first time that it was using plutonium yielded from the rods to make nuclear weapons.
U.S. and South Korean officials have been skeptical about the claims that the rods have been reprocessed.
On Thursday, however, a senior Seoul official, speaking to South Korean reporters on condition of anonymity, said South Korea was investigating the latest claim "while keeping all possibilities open."
Foreign Ministry spokesman Shin Bong-kil urged North Korea to refrain from any steps that would worsen the situation and to return to six-nation talks on its nuclear programs.
"We express our concern that this latest North Korean statement could hurt efforts to resolve the nuclear problem peacefully, hurt development of South-North Korean relations and damage the atmosphere of dialogue," Shin said in a statement.
China's state-run Xinhua News Agency reported Thursday that North Korean Vice Foreign Minister Choe Su Hon insisted Pyongyang's nuclear capability was "not intended to attack other countries."
Choe, in New York for a meeting of the U.N. General Assembly, also said: "We (have) no intention of transferring any means of that nuclear deterrence to other countries."
On Thursday, North Korea also said that when necessary, it will reprocess more spent fuel rods to be produced from the small reactor in its main nuclear complex in Yongbyon, 50 miles north of Pyongyang.
North Korea says its has restarted its frozen 5-megawatt reactor at Yongbyon after it kicked out U.N. nuclear inspectors and quit the Nuclear Nonproliferation Treaty in January. Experts say it would take a year of operation before the reactor can produce enough to make a new weapon.
The United States and its allies are trying to persuade North Korea to give up its nuclear programs. North Korea says it will do so only if the United States signs a nonaggression treaty, provides economic aid and opens diplomatic ties.
The United States, the two Koreas, China, Japan and Russia met in Beijing in August to defuse the crisis. The meeting ended without agreement on when to hold the next round as Washington and Pyongyang differed widely over how to resolve the dispute.
North Korea has since said it was no longer interested in further talks, which the North's last remaining major ally, China, has been working to restart.
North Korea tends to escalate its harsh rhetoric in attempts analysts say are aimed at extracting concessions in crucial negotiations.
Japan and China did not comment on the North Korean statement Thursday, but it drew expressions of concern from other Asian governments.
"Any steps that bring nearer the prospect of nuclear proliferation on the Korean peninsula would be a source of great concern to Indonesia," Indonesian Foreign Ministry spokesman Marty Natalegawa said.
Philippine Foreign Affairs Undersecretary Franklin Ebdalin said it was unfortunate and would make the nuclear standoff "more difficult to resolve."
The nuclear dispute flared last October when U.S. officials said North Korea admitted running a secret nuclear weapons program in violation of international agreements.