VIENNA (Reuters) - United Nations nuclear inspectors have found uranium particles refined to a higher-than-expected level at an underground site where Iran has installed more than 50 percent more enrichment centrifuges, a U.N. watchdog report said on Friday.
It said Tehran had told the U.N. agency that the presence of traces of highly refined uranium - still well below potential nuclear weapons-grade material - "may happen for technical reasons beyond the operator's control".
The United States - which like its Western allies and Israel accuse Iran of seeking to develop atomic bomb capability - said the Iranian explanation could be correct and a leading U.S. expert said he saw nothing "nefarious" in the discovery.
The International Atomic Energy Agency report came a day after six world powers - the United States, Russia, Britain, Germany, China and France - failed to convince Iran to halt its most sensitive nuclear work during May 23-24 talks in Baghdad.
At the heart of the dispute is Iran's insistence on a right to enrich uranium and that economic sanctions should be lifted before it shelves activities that could lead to its achieving the ability to assemble nuclear weapons.
Western powers insist Tehran must first shut down higher-grade enrichment before sanctions could be eased.
Iran started enriching to a fissile concentration of 20 percent in 2010 and has since sharply expanded the activity, saying the material will serve as fuel for a medical reactor.
But a suspicious West is alarmed since such enhanced enrichment accomplishes much of the technical leap towards 90 percent - or weapons-grade - uranium.
The IAEA report said environmental samples taken in February at Iran's Fordow facility - buried deep beneath rock and soil to protect it from air strikes - showed the presence of particles with enrichment levels of up to 27 percent.
That is above the 20 percent enrichment level Iran has declared at the site, and takes it across the line from low-enriched to high-enriched uranium.
"The Agency is assessing Iran's explanation and has requested further details," the IAEA report said.
In Washington, U.S. State Department spokeswoman Victoria Nuland said: "There are a number of possible explanations for this, including the one that the Iranians have provided. We are going to depend on the IAEA to get to the bottom of it."
A diplomat familiar with the issue said a "number" of such particles had been discovered at Fordow and that further samples were taken this month to see whether the find was confirmed.
U.S. proliferation expert David Albright said it was "embarrassing for Iran" but he saw no reason for concern.
"I think they just did it as they were starting up the cascade (network of centrifuges). It is nothing special. It is not nefarious," he told Reuters.
The IAEA report suggested it is possible that particles of uranium enriched to higher-than-declared levels may be the result of a technical phenomenon associated with the start-up of centrifuge cascades.
Enriched uranium can be used to fuel power plants, which is Iran's stated purpose, or provide material for bombs, if refined to a much higher degree, which the West suspects may be Iran's ultimate goal. The Islamic Republic denies that.
Iran has increased its stockpile of 20 percent uranium to around 145 kg in May from nearly 110 kg some three months ago, the IAEA report said. Western experts say about 250 kg is needed for a nuclear bomb, if processed further.
The report showed the Islamic state had installed 368 enrichment centrifuges at Fordow in addition to the 696 already operating there, and had expanded lower-level enrichment at its main enrichment site at Natanz in central Iran.
Although not yet being fed with uranium, the new machines could be used to further boost Iran's output of uranium enriched to 20 percent, the part of the country's nuclear programme that most worries the West.
Albright said Iran was improving its skills at enriching and that the monthly production rate of low-enriched uranium had jumped "a great deal" at Natanz.
The U.N. agency also said satellite images showed "extensive activities" at the Parchin military complex, southeast of Tehran, which inspectors want to check over suspicions that nuclear weapons-relevant research was done there.
The "activities" could hamper the IAEA's inquiry, it said - an allusion to what Western diplomats have said may be Iranian efforts to remove incriminating evidence. Iran has denied pursuing a clear weapons capability there or anywhere else.
The U.N. watchdog told Iran in a letter sent this month that it needed "early access" to Parchin, the report said. Iran has so far refused this, saying that first a broader deal with the IAEA on how to clarify the agency's concerns must be finalised.
IAEA chief Yukiya Amano said earlier this week after talks in Tehran that the two sides were close to an agreement to enable inspectors to resume their long-running investigation into suspected nuclear explosive experiments in Iran, although "some differences" remained before it could be sealed.
(Additional reporting by Arshad Mohammed in Washington; Editing by Michael Roddy)