VIENNA – Inspectors have located radioactive traces at an Iranian underground bunker, the U.N. atomic agency said Friday — a finding that could mean Iran has moved closer to reaching the uranium threshold needed to arm nuclear missiles.

In a report obtained by The Associated Press, the International Atomic Energy Agency said it was asking Tehran for a full explanation. But the report was careful to avoid any suggestion that Iran was intentionally increasing the level of its uranium enrichment, noting that Tehran said a technical glitch was responsible.

Analysts as well as diplomats who had told the AP of the existence of the traces before publication of the confidential report also said the higher-enriched material could have been a mishap involving centrifuges over-performing as technicians adjusted their output rather than a dangerous step toward building a bomb.

Still, the finding was bound to resonate among the 35 IAEA board members for whom the report was prepared, among them the six world powers that had just concluded talks with Iran on its enrichment activities. The negotiations in Baghdad left the two sides still far apart over how to oversee Tehran´s atomic program but resolved to keep dialogue going next month in Moscow as an alternative to possible military action.

The report also expressed in opaque terms what diplomats accredited to the IAEA first started mentioning more than a month ago: The agency fears a massive cleanup under way at buildings at the Parchin military complex southeast of Tehran. The site is suspected of housing a pressure chamber and related equipment used to test ways of detonating a nuclear charge.

"Based on satellite imagery at this location ... the buildings of interest to the agency are now subject to extensive activities that could hamper the agency´s ability to undertake effective verification" at the site, said the report. Separately, a senior international official familiar with the issue said satellite photos showed trucks at the site and streams of liquid suggesting the interiors were being hosed down to wash away evidence of possible nuclear-related work.

The report said that over the past five months the agency had "obtained more information" buttressing its suspicions about the Parchin site, which it has asked repeatedly to visit, only to be turned down by Iran.

On enrichment, the six world powers trying to engage Iran are already concerned about its output of uranium at the 20 percent level because material that high can be turned into weapons-grade much more quickly than its main, low-enriched stockpile.

The higher the enrichment, the easier it becomes to re-enrich uranium to warhead quality at 90 percent. As a result, the finding of traces at 27 percent at the Fordo enrichment plant in central Iran sparked international interest.

Iran denies any plans to possess nuclear weapons but has for years declined offers of reactor fuel from abroad, including more recent inducements of 20-percent material if it stops producing at that level. The Islamic Republic says it wants to continue producing 20 percent uranium to fuel its research reactor and for medical purposes.

But its refusal to accept foreign offers has increased fears it may want to turn its enrichment activities toward producing such arms. The concerns have been fed by IAEA suspicions that Iran has experimented on components of an atomic arms program — suspicions Tehran also denies.

The report cited a May 9 letter from Iranian officials suggesting any enrichment at 27 percent was inadvertent. The letter said the particles were produced "above the target value" and could have been for "technical reasons beyond the operator´s control."

David Albright, whose Washington-based Institute for Science and International Security looks for signs of proliferation, said a new configuration at Fordo means it tends to "overshoot 20 percent" at the start.

"Nonetheless, embarrassing for Iran," he wrote in an email to the AP.

Mark Fitzpatrick, director of the Non-Proliferation and Disarmament Program of the International Institute for Strategic Studies, described such a surge as a "naturally occurring development," adding that "it´s very usual" to go above envisaged enrichment levels at startups of centrifuges.

In Washington, State Department spokeswoman Victoria Nuland said "there are a number of possible explanations for this, including the one that the Iranians have provided. But we are going to depend on the IAEA to get to the bottom of it."

Others were more skeptical.

"It´s not surprising because they have the technology," a senior Israeli defense official said.

"Iran doesn´t intend to stop its nuclear weapon program, and the fact that they are at 27 percent shows the Iranian intentions," said the official, who spoke on condition of anonymity because he is not allowed to speak with the media.

International concerns have increased since Iran started higher enrichment at Fordo, which is carved into a mountain to make it impervious to attack. Israel and the United States have not ruled out using force as a last option if diplomacy fails to curb the Islamic Republic´s nuclear program.

Iran already has about 700 centrifuges churning out 20-percent enriched uranium at Fordo. The report noted that although Iran has set up about 350 more centrifuges since late last year at the site, these machines are not enriching.

While the reason for that could be purely technical, it could also be a signal from Tehran that it is waiting for progress in the negotiations.

The latest attempt to persuade Iran to compromise ended inconclusively Thursday after two days of talks in Baghdad. The six world powers — the United States, Russia, China, Britain, France and Germany — failed to persuade Tehran to freeze its 20 percent enrichment.

Iran, for its part, failed to persuade the West to scale back on recently toughened sanctions, which have targeted Iran´s critical oil exports and have effectively blackballed the country from international banking networks. The 27-nation European Union is set to ban all Iranian fuel imports on July 1, shutting the door on about 18 percent of Iran´s market.

But the IAEA report did detail some progress in separate talks between the U.N. nuclear agency and Iran that the agency hopes will re-launch a long-stalled probe into the suspicions that Tehran has worked on nuclear-weapons related experiments.

The report said that since 2002, the IAEA "has become increasingly concerned about the possible existence in Iran of undisclosed nuclear- related activities involving military-related organizations, including activities related to the development of a nuclear payload."

Without more openness on the part of Iran, said the report, the IAEA "cannot conclude that all nuclear material in Iran is in peaceful activities." ___ AP writer Ian Deitch in Jerusalem contributed this report. (© 2012 The Associated Press 05/25/12)