Top nuclear envoys from South Korea and the United States held talks Saturday on a strategy to bring North Korea back to disarmament negotiations, a day after the North claimed to have succeeded in experimental uranium enrichment. US special envoy on North Korea, Stephen Bosworth, and South Korean envoy Wi Sung-lac made no comments after their meeting. Bosworth later met with South Korea's unification minister in charge of relations with North Korea. Bosworth said in Beijing on Friday that any nuclear development in North Korea was a matter of concern. "We confirm the necessity to maintain a coordinated position and the need for a complete denuclearization of the Korean peninsula," he said. North Korea's claim that it is in the final stages of enriching uranium raises the possibility that it might be seeking to refresh its stockpile of bombs made from plutonium. Uranium offers an easier way to make nuclear weapons. North Korea also said it is continuing to weaponize plutonium. Washington shows no signs of easing pressure on North Korea through sanctions, although the North has also recently made a series of conciliatory gestures, including the release of two detained American journalists and a reported invitation to top US envoys, including Bosworth, to visit Pyongyang. "We are prepared for both dialogue and sanctions," the North said in a letter to the UN Security Council carried Friday by its official Korean Central News Agency. If some members of the council put "sanctions first before dialogue, we would respond with bolstering our nuclear deterrence first before we meet them in a dialogue," it said. The North warned it would be left with no choice but to take "yet another strong self-defensive countermeasure" if the standoff continues. It did not elaborate. A pro-North Korean newspaper in Japan urged the US to hold talks with the North to make the Korean peninsula nuclear-free, saying Pyongyang's next steps will depend on how Washington reacts to its latest moves. The Choson Sinbo newspaper, widely seen as a mouthpiece for North Korea, said time is not "limitless" for the US to decide whether to hold talks or continue to pursue sanctions. US State Department spokesman Ian Kelly said the North's announcement Friday was troubling. "We are very concerned by these claims that they are moving closer to the weaponization of nuclear materials, but I can't really comment on the veracity, how true these claims are," Kelly said. The US and other Security Council members have been focusing on implementation of a resolution that imposed sanctions on the North's weapons exports and financial dealings, and allowed inspections of suspect cargo in ports and on the high seas. The US has pressed for North Korea to return to six-nation talks on its nuclear program. The North pulled out of the negotiations with the US, South Korea, China, Russia and Japan after the council criticized its April rocket launch. North Korea said later it won't return to the negotiations and will only talk one-on-one with the Obama administration. Bosworth said Friday that the US is willing to have direct talks with the North, but only within the framework of the six-nation disarmament talks. Analysts said the North appears to be trying to add urgency to the standoff to get Washington into one-on-one negotiations. "I think this is a 'let's-have-direct talks' message," said Yang Moo-jin, a professor at Seoul's University of North Korean Studies. "The North is saying that the more delayed US-North Korea talks are, the greater its nuclear capabilities will become." Meanwhile, the Washington-based Institute for Science and International Security said in a report that there was no sign of reconstruction at the North's main Yongbyon nuclear reactor, which was partially disabled under an agreement reached in the six-nation talks. It cited commercial satellite imagery taken on August 10 by DigitalGlobe. Bosworth, who met Chinese officials in Beijing, is to leave for Tokyo on Sunday for similar consultations with Japanese officials. Chief US nuclear negotiator, Sung Kim, who is accompanying Bosworth, plans to return to Seoul on Tuesday to meet with Russian nuclear envoy Grigory Logvinov. Uranium can be enriched in relatively inconspicuous factories and could provide North Korea with an easier way to build nuclear bombs, experts say. Uranium-based bombs may also work without requiring test explosions like the two carried out by North Korea this May and in 2006 for plutonium-based weapons. Separately, North Korea also said it will continue to seek self-defensive measures in response to an alleged US move to develop a new bunker-buster bomb, the official KCNA news agency reported. It claimed the US is accelerating production of the bomb to destroy "underground nuclear facilities" in North Korea and Iran.