SEOUL: 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.

U.S. special envoy on North Korea Stephen Bosworth and South Korean envoy Wi Sung-lac made no comments after their meeting. 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 enrichment — an easier way to make nuclear weapons — raises the possibility that it might add uranium-based weapons to its stockpile of bombs made from plutonium.

The tough talk came as Washington showed no signs of easing pressure on North Korea through sanctions despite the North's recent series of conciliatory gestures, including the release of two detained American journalists and a reported invitation to top U.S. envoys, including Bosworth, to visit Pyongyang.

"We are prepared for both dialogue and sanctions," the North said in a letter to the U.N. 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 Security Council slapped tough sanctions on North Korea for conducting an underground nuclear test in May.

The North said it does not oppose the denuclearization of the Korean peninsula, but 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 on the possible countermeasures.

The letter stressed "the denuclearization of the Korean peninsula is closely related with the U.S. nuclear policy" toward North Korea.

U.S. State Department spokesman Ian Kelly said the North's announcement 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 U.S. and other Security Council members have been focusing on implementation of a resolution adopted unanimously in June after the North conducted its nuclear test and a suspected ballistic missile launch. The resolution imposed tough new sanctions on the North's weapons exports and financial dealings, and allowed inspections of suspect cargo in ports and on the high seas.

The U.S. has pressed for North Korea to return to six-nation talks on its nuclear program. The North pulled out of the negotiations with the U.S., 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 on Friday that the U.S. 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 U.S.-North Korea talks are, the greater its nuclear capabilities will become."

Bosworth, who met Chinese officials in Beijing, is to leave for Tokyo on Sunday for similar consultations with Japanese officials. Chief U.S. 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 pluto