U.S. Assistant Secretary of State Christopher Hill, Washington's delegate to the North Korean nuclear talks, had blunt words for Pyongyang on Monday.

"We are also focusing on the fact that we see North Korea stalling. And we see North Korea not as committed to the talks as the rest of us are," he said. "We need to be very clear that it is not acceptable for them to be staying out of the talks."

Mr. Hill met with South Korean Foreign Minister Ban Ki-moon in Seoul Monday. He is scheduled to travel to Beijing and Tokyo later this week, to discuss North Korea's refusal to take part in multi-lateral talks aimed at ending its nuclear programs. He says the U.S. is working in close partnership with its allies in Seoul.

"We have a lot to discuss together," said Ambassador Hill. "I'd say we have a very good understanding of this issue, and very good agreement on the best tactics to bring this issue to a resolution."

North Korea has boycotted disarmament talks since last June. The country says it already possesses nuclear weapons, and a senior North Korean military official recently repeated threats to "steadily bolster" the nuclear arsenal to counter what Pyongyang calls hostility by Washington.

Pyongyang has reportedly shut down its Yongbyon nuclear reactor - a possible sign of plans to reprocess spent fuel rods into weapons-grade material.

It is not clear how many nuclear weapons North Korea might have, if any at all, and Pyongyang has never given figures. U.S. officials have said they suspect it might possess one or two such weapons.

South Korean Foreign Minister Ban earlier Monday warned North Korea not to take "reckless measures, such as nuclear tests." The statement was an apparent response to U.S. media reports quoting American officials as saying Pyongyang might be preparing a nuclear test.

Mr. Ban warned the North it would be risking further isolation if it took such actions.