North Korea poses the "most immediate and most serious threat" to efforts to control the world's nuclear weapons, the U.N. atomic watchdog agency's chief warned Friday.

Mohamed ElBaradei (search), director-general of the International Atomic Energy Agency (search), said he was concerned about the latest reports that Pyongyang is reprocessing fuel rods that were under his agency's safeguards.

ElBaradei also pressed Iran for "substantial progress without delay" in clarifying aspects of its nuclear program and in signing an agreement that would let U.N. inspectors conduct in-depth and comprehensive checks of Tehran's nuclear facilities.

U.S. officials say they're not sure whether North Korean representatives were bluffing or telling the truth when they claimed last week to have finished extracting plutonium (search) -- a key ingredient for nuclear weapons -- from 8,000 spent nuclear fuel rods.

"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," he said. "I earnestly hope that the international community will urgently focus its efforts on bringing the DPRK back to the nonproliferation regime."

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, who made the remarks in a statement issued after the agency's board of governors met in Vienna to approve a new budget, said he was "committed to continuing to work with all concerned parties to help achieve a comprehensive solution to this problem."

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 later be replaced by five-way multilateral talks that also would include South Korea and Japan, they said.

The nuclear dispute flared in October when North Korea reportedly told a top U.S. official it had restarted a nuclear program in violation of a 1994 accord.

The United States and its allies suspended fuel shipments promised under the 1994 deal, and Pyongyang retaliated by expelling U.N. monitors, restarting frozen nuclear facilities and withdrawing from the Nuclear Nonproliferation Treaty (search).

In December, North Korea expelled nuclear inspectors from the Vienna-based International Atomic Energy Agency, shortly after it had dismantled U.N. seals and monitoring cameras installed at the country's nuclear facilities.

The facilities had been mothballed under the 1994 agreement with the United States.