The Iranians were speaking at a technical briefing of the Vienna-based International Atomic Energy Agency, where IAEA officials said the Iranians had indicated to international black marketeers that they would be interested in buying tens of thousands of magnets for sophisticated P-2 centrifuges that can enrich uranium to bomb-grade levels, the diplomats said.
Washington charges that Iran is secretly trying to develop atomic weapons and should be taken to the UN Security Council for possible sanctions.
Iran's clerical regime insists its nuclear programme is for peaceful purposes.
The IAEA's 35-nation board of governors is to meet on Iran next week and is expected to rap the Islamic Republic for hiding sensitive atomic activities but avoid provoking a showdown over the country's alleged secret weapons programme.
Tens of thousands of magnets would be enough to make production-line cascades of thousands of centrifuges to manufacture raw material for nuclear bombs.
"At two magnets per P-2 centrifuges, that's quite a few centrifuges," a Western diplomat said.
Two thousand P-2 centrifuges can produce enough highly enriched uranium (HEU) for two nuclear devices per year, experts said.
Diplomats said the Iranians, who claim they are only doing research into centrifuges rather than trying to start large-scale manufacture of HEU, had told the IAEA they were inquiring about such a large purchase of magnets only to get the price down.
Iranian officials confirmed at the briefing that agency inspectors had found particles of uranium enriched to 54 percent, the diplomats said.
The previous highest level made public in its ongoing Iran investigation was 36 percent.
"There was a silence in the room when they mentioned 54 percent," said one diplomat, who asked not to be named. "People were shocked by the figure."
Uranium enriched to over 20 percent can be used to make a nuclear bomb but most nuclear weapons are made with levels of over 80 percent enrichment.
Diplomats said an IAEA chart at the briefing indicated contamination in some cases of close to 80 percent.
IAEA officials said they were so far unable to determine the source of the
An Iranian official told reporters after the meeting that American "misunderstandings" about Tehran's nuclear programme had been cleared up, but a US diplomat denied this.
"Mainly the US had a number of misunderstandings that we hope have now been made clear," Iranian foreign ministry official Amir Zamaninia told journalists after the closed-door technical meeting.
But US ambassador to the IAEA Kenneth Brill said he had told the IAEA "that in the official written statement which the Iranians issued on March 5 about the P-2 program about their centifuge program there were at my count at least four errors of accounting that were corrected by the most recent report" of IAEA director general Mohamed ElBaradei on Iran.
"I didn't hear anything that corrected that understanding although I did hear an effor to try to explain it away," Brill said.
A tough Washington-inspired IAEA board resolution in March that condemned Iran for omitting to report its work into sophisticated P-2 centrifuges led Tehran to delay crucial agency investigations.
The United States looks ready to sign on this time to a British-French-German draft resolution that sharply criticizes Iran for failing to answer questions about alleged nuclear weapons activities but presses for continued cooperation with Tehran, diplomats said.
At their meeting in Sea Island in the US state of Georgia on Wednesday, G8 leaders chastised Iran for allegedly failing to fully disclose its nuclear programme.