North Korea said Friday that it is in the final stages of enriching uranium, a process that could give it a second way to make nuclear bombs in addition to its known plutonium-based program. North Korean state media said officials had informed the UN Security Council it is forging ahead with its nuclear programs in spite of international calls to abandon them. "Reprocessing of spent fuel rods is at its final phase and extracted plutonium is being weaponized. Experimental uranium enrichment has successfully been conducted to enter into completion phase," the Korean Central News Agency reported. The US and North Korea's neighbors had been negotiating for years with the North to dismantle its plutonium-based nuclear program, which experts say has yielded enough weaponized plutonium for at least half a dozen atomic bombs. North Korea walked away from those talks earlier this year. The US had long suspected that the North also had a covert uranium enrichment program, which would give it a second source of nuclear material. North Korea for years denied the claim but in response to UN sanctions announced in June that it could enrich uranium. The United States' special envoy on North Korea said any activities in the area of nuclear development are "of concern to us." "These are issues we are dealing with as they arise and we maintain the need for cooperation and dialogue and complete denuclearization" of the Korean peninsula, Stephen Bosworth said in Beijing during an Asia trip to discuss how to bring North Korea back to disarmament talks. Enriched uranium would provide the North with an easier way to build nuclear bombs compared to reprocessing plutonium. Uranium also can be enriched in relatively inconspicuous factories that are better able to evade spy satellite detection, according to experts in the US and at South Korea's Institute of Nuclear Nonproliferation and Control. Uranium-based bombs may also work without requiring test explosions like the two carried out by North Korea in May and in 2006 for plutonium-based weapons. However, plutonium bombs have more potential to be miniaturized to fit on top of a missile, according to one expert, Ivan Oelrich of the Federation of American Scientists. North Korea is not believed to have mastered making a nuclear bomb small enough to mount in a long-range missile. An April rocket launch widely condemned by international powers and the UN Security Council was seen by many as a test of its long-range missile technology. It won't be easy to verify North Korea's claims on uranium enrichment, South Korean Defense Ministry spokesman Won Tae-jae said, adding that the announcement could just be a negotiating tactic. Friday's statement comes amid a series of conciliatory moves after months of provocation, from the April rocket launch and May nuclear test to the test-firing of a flurry of missiles. North Korea called its actions a response to the Security Council's decision to tighten sanctions against the regime as punishment for the May nuclear test, a resolution KCNA called a "wanton violation" of the country's sovereignty. The US, China, Japan, Russia and South Korea have been trying for years to persuade North Korea to dismantle its nuclear program in exchange for much-needed aid and other concessions. The North says it needs the nuclear program as a security guarantee against a threat from the US, which has 28,500 troops based in South Korea, which technically remains at war with the North because their three-year conflict in the early 1950s ended in a truce, not a peace treaty. "We are prepared for both dialogue and sanctions," the KCNA report said, warning it would be left with no choice but to take "yet stronger self-defensive countermeasures" if the standoff continues. It did not elaborate on the possible countermeasures. South Korea's Foreign Ministry expressed regret and urged the communist country to abandon its nuclear weapons programs and return to the stalled disarmament talks. "The North's move to continue provocative steps ... can never be tolerated. We will deal with North Korea's threats and provocative acts in a stern and consistent manner," it said in a statement. In Tokyo, Japanese Foreign Minister Hirofumi Nakasone urged North Korea to refrain from actions and remarks that could heighten tensions. "We will definitely not tolerate North Korea possessing nuclear weapons," Nakasone said, adding that Japan is cooperating closely with South Korea, the US and China on North Korean issues. Bosworth, the US envoy, met Thursday and Friday with Chinese officials. "We confirmed that the US and China share a strong common approach in dealing with the challenges of North Korea," he said. "We agree on the complete and verifiable denuclearization of the North Korean peninsula, which remains our core objective." Bosworth, who heads next to Seoul and Tokyo, reiterated the importance of the six-nation framework for any talks, including those between the US and North Korea, and emphasized President Barack Obama's commitment to dialogue with North Korea. The North has long sought one-on-one negotiations with Washington. The US has said it is willing to hold direct talks, but under the framework of the six-nation disarmament negotiations. KCNA said Friday that North Korea has never objected to the denuclearization of the Korean peninsula, and left open possibility for dialogue with some permanent members of the Security Council - an apparent reference to the US In promising signs, North Korea last month freed two jailed American journalists and five detained South Koreans. The two Koreas also agreed to restart the temporary reunions of families separated by the 1950-53 Korean War, and restored regular cross-border traffic to a joint industrial park in the North.