Mohamed ElBaradei, director-general of the International Atomic Energy Agency, said he was concerned about the latest reports that Pyongyang is reprocessing fuel rods that were under his agency's safeguards.
"In my view, the situation in the DPRK is currently the most immediate and most serious threat to the nuclear nonproliferation regime," ElBaradei said, referring to the acronym for the Democratic People's Republic of Korea.
"I find it regrettable that little concrete progress on the issue appears to have been made since December, when the agency's verification work came to a halt. I earnestly hope that the international community will urgently focus its efforts on bringing the DPRK back to the nonproliferation regime."
North Korea expelled international inspectors in December, and the United States relies mainly on satellite images for clues about what is going on at the communist nation's nuclear facilities.
ElBaradei said, however, that he was "encouraged by some recent efforts on the part of China to restart a dialogue" toward persuading the North to abandon its weapons program.
The IAEA chief said he was "committed to continuing to work with all concerned parties to help achieve a comprehensive solution to this problem."
Tensions on the Korean peninsula have been rising since October, when Pyongyang admitted having a clandestine program to enrich uranium for nuclear weapons.
The admission led to the collapse of the 1994 agreement in which North Korea had agreed to freeze its nuclear weapons program in exchange for fuel aid and the construction of two nuclear power generating stations.
Since then, North Korea has withdrawn from the Nuclear Non-Proliferation Treaty and ejected International Atomic Energy Agency inspectors.
North Korean representatives claimed last week to have finished extracting plutonium from 8,000 spent nuclear fuel rods. That could allow the communist country to build six nuclear bombs, adding to an arsenal suspected of already holding one or two nuclear weapons.
President Bush has vowed not to tolerate nuclear weapons on the Korean peninsula. Besides threatening the 37,000 American troops in South Korea, U.S. officials worry North Korea could sell or give some of its nuclear material — or worse, an assembled weapon — to other states or to terrorists.
White House and Pentagon officials said North Korea's announcement that it is moving toward producing new nuclear weapons was a problem, but not a crisis. The White House says that while he has not ruled out a military response, Mr. Bush plans to continue pressing for a diplomatic solution to the impasse.
But former Defense Secretary William Perry said this week in an interview with The Washington Post: "I have thought for some months that if the North Koreans moved toward processing, then we are on a path toward war."
The United States has insisted on talks involving other countries in the region, such as China, South Korea and Japan, while North Korea has for the most part said it wants to talk only to the United States.
South Korean news reports say China is pushing for a new round of three-way talks, involving North Korea, the United States and China. The format would be later replaced by five-way multilateral talks that will also include South Korea and Japan, they said.