Vienna, Austria ? The U.N. atomic agency’s chief called North Korea “the most serious threat” to nuclear proliferation Friday while urging Iran to let inspectors investigate suspect nuclear facilities.

Mohamed ElBaradei, director-general of the International Atomic Energy Agency, said he was concerned about reports that North Korea was reprocessing fuel rods.

“We are not there,” he said. “We would like to be there.”

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.

Though he said the Vienna-based agency had amassed results from its inspections of the Iranian facilities, he denied reports that experts determined 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 said. “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.”

In Washington, State Department spokesman Richard Boucher said the United States expected a “full and factual accounting” of the agency’s findings in Iran, hopefully “well in advance” of the next scheduled meeting of the agency’s board in early September.

The United States wants the agency to declare Iran in violation of the Nuclear Nonproliferation Treaty, making it possible to send the matter to the U.N. Security Council for possible action. The council has so far declined to take any action on North Korea.