The UN atomic agency's chief called North Korea "the most serious threat" to nuclear proliferation, expressing concern over reports that Pyongyang is reprocessing fuel rods.
The North Koreans last week claimed to have finished extracting plutonium -- a key ingredient for nuclear weapons -- from 8,000 spent nuclear fuel rods.
Mohamed ElBaradei, director-general of the International Atomic Energy Agency, urged renewed international efforts to pressure North Korea to honor the treaty designed to stop the spread of nuclear weapons.
"In my view, the situation in the DPRK is currently the most immediate and most serious threat to the nuclear non-proliferation regime," ElBaradei said, referring to the acronym for the Democratic People's Republic of Korea.
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.
China sent Vice Foreign Minister Dai Bingguo (戴秉國), Beijing's most experienced envoy on North Korean affairs, to Pyongyang last weekend.
China's UN Ambassador Zhang Yishan told UN Secretary-General Kofi Annan on Friday that the visit yielded "encouraging results."
Dai was in Washington to brief top US officials, including Secretary of State Colin Powell, US security adviser Condoleezza Rice and Vice President Dick Cheney, on the trip and to discuss starting new talks with Pyongyang.
The Chinese envoy described Friday's sessions as useful, saying, "We both agreed to work together to push the process forward and resolve the issue."
Dai did not elaborate, and US officials had no immediate comment on the talks.
The dispute between Pyongyang and Washington emerged last fall when North Korea reportedly told a top US official it had restarted a nuclear program in violation of a 1994 accord.
The US and its allies suspended fuel shipments promised under the 1994 deal, and Pyongyang retaliated by expelling UN monitors, restarting frozen nuclear facilities and withdrawing from the Nuclear Nonproliferation Treaty.
North Korea expelled nuclear inspectors from the UN nuclear watchdog agency in December, shortly after it had dismantled UN seals and monitoring cameras installed at the country's nuclear facilities. The facilities had been mothballed under the 1994 agreement with the US.
ElBaradei also pressed Iran for "substantial progress without delay" in clarifying aspects of its nuclear program and in signing an agreement that would let UN inspectors conduct in-depth and comprehensive checks of Tehran's nuclear facilities.
Though he said the Vienna-based IAEA has amassed results from its ongoing inspections of the Iranian facilities, he denied reports that experts had determined that enriched uranium was found in the samples. He described the reports as "pure speculation at this stage."
"There's a lot of analysis we need to discuss, a lot of results with Iran," ElBaradei told reporters. "We are not in any way ready to come up with a conclusion on that issue before we discuss all the results with the Iranian authorities."
Meanwhile, North Korea has deployed additional long-range missiles capable of reaching Japan, the South Korean Defense Ministry said yesterday, adding to concerns over Pyongyang's nuclear program.
With tensions high on the peninsula, the ministry also said North Korean artillery had been brought closer to Seoul.