International atomic inspectors in Iran have detected traces of uranium enriched to levels of purity higher than the Iranians have previously disclosed, according to a new report on Tehran’s nuclear program made public on Friday.

The report, by the International Atomic Energy Agency, an arm of the United Nations based in Vienna, said its inspectors had taken environmental samples at a uranium-enrichment plant in a mountain bunker and discovered purities up to 27 percent.

While the report suggests that the finding could be an innocuous aberration, it is potentially significant because it moves Iran’s uranium enrichment closer to bomb-grade purity, even as world powers are in the midst of intensive negotiations with Tehran to go in the opposite direction.

The report said Iran had sought to explain the spike, found in February at its once-secret Fordo enrichment plant near the holy city of Qum, as possibly resulting from “technical reasons beyond the operator’s control.” But the report also said inspectors were doing more checking.

Diplomats and nuclear experts said the rise appeared in fact to reflect honest technical missteps rather than evidence that Iran had embarked on secret enrichments at higher purities.

“It’s definitely embarrassing but not nefarious,” David Albright, president of the Institute for Science and International Security, a Washington research group that tracks the Iranian nuclear program, said in an interview.

A senior Obama administration official agreed that “the most likely explanation” for the discovery was technical. If the energy agency had found uranium enriched to 30 percent or 60 percent, the official said, it would be greater cause for concern.

The report, by the agency’s director general, Yukiya Amano, was one of his quarterly briefings to its board and the United Nations Security Council, which has placed sanctions on Iran four times for its refusal to halt its uranium enrichment.

The West suspects that Iran wants to amass the uranium for the capability to build atomic bombs, while Iran insists it wants only to fuel civilian reactors. The standoff has gone on for a decade, with the geopolitics and technical issues ever more complex.

The disclosure of uranium particles enriched up to 27 percent came less than a day after Iran and a group of six world powers ended a round of difficult talks in Baghdad on Iran’s nuclear program. While diplomats reported no substantive progress, both sides agreed to meet again in Moscow next month.

Mr. Amano’s quarterly report said that the atomic agency was assessing Iran’s explanation for the enrichment spike and that inspectors earlier this month had taken “further environmental samples from the same location where the particles in question had been found.”

In Vienna, a diplomat, speaking about the enrichment increase on the condition of anonymity, said that “there’s a decent chance that it’s an operator error.”

Until now, the highest reported level of uranium enrichment by the Iranians has been 20 percent. The Nuclear Nonproliferation Treaty makes no restrictions on levels of uranium enrichment, only barring nations from turning their civilian efforts to military ends.

In Iran’s case, the United Nations sanctions and the rising global concern have been prompted by evidence that Iran has pursued the making of nuclear arms despite its denials and a decree by the supreme leader, Ayatollah Ali Khamenei, that such weapons are forbidden under Islam.

Uranium fuel for most reactors is enriched to around 4 percent purity. Two years ago, Iran began producing uranium enriched to 20 percent, saying it was for a research reactor in Tehran.

Mr. Amano’s quarterly report documented that Iran was indeed turning some of its growing stockpile of 20 percent uranium into fuel for the reactor.

Still, major questions remain about the fate of Iran’s stockpile. Twenty percent enrichment is a short technical step from 90 percent, the purity needed to make an atomic bomb. The quarterly report also said the atomic agency had gathered information that “further corroborates” its analysis that Iran might have performed experiments related to the design of nuclear arms at Parchin, a sprawling military complex 20 miles southeast of Tehran. Rick Gladstone contributed reporting from New York, and Mark Landler from Washington. (Copyright 2012 The New York Times Company 05/26/12)