The diplomats said the particles of weapon-grade uranium came from sample swipes inspectors had taken last January at the Lavizan Shian site in Tehran. The Iranians dismantled a physics research centre at the site and removed topsoil in 2004 after suspicions were raised about activities there.
"They have found particles of HEU (highly enriched uranium) but it is not clear if this is contamination from centrifuges that had been previously found (from imported material) or something new," said one diplomat close to the UN International Atomic Energy Agency (IAEA), referring to the inspectors.
Two other diplomats said the HEU, which is in microscopic amounts, was from vacuum pumps that could be used in a centrifuge, adding that the Iranians were nervous about the findings.
"There is something there," said one diplomat, who did not provide further details and like the others asked not to be named due to the sensitivity of the issue.
Iran denied the accusation, Foreign Ministry spokesman Hamid Reza Asefi telling state television that the latest claims were "baseless and without importance."
"These declarations do not have any basis in truth but look like previous assertions that were later denied (by the IAEA)”, Mr Asefi added.
IAEA inspectors have sought to investigate machines that were used at the Lavizan site, which could be for either civilian or weapons purposes, but in a report late last month the watchdog said it had not yet received clarification from Iranian authorities on the machinery.
Inspectors had previously found HEU particles as well as low enriched uranium, which can be used for nuclear fuel but is not refined enough for weapons use, on centrifuge equipment at several sites in Iran.
The Iranian government has denied those traces were from its program, saying that they were contamination from equipment acquired from in Pakistan.
Significant finding
Non-proliferation expert David Albright said the reported find was "significant” the President of the Institute for Science and International Security think tank adding that, "It could be part of Iran's known centrifuge program or a parallel (hidden) program."
Iran says its nuclear program is a peaceful drive to generate electricity, but some in the international community chief among them the United States, are concerned that those claims are a cover for the secret development of atomic weapons.
The IAEA has been investigating Iran since 2003 and says it is not yet able to certify that the Iranian nuclear program is strictly peaceful.
Since April the 11th Iran has been enriching uranium at a centrifuge cascade in Natanz but only to levels of up to five percent, which is far below the 20 percent enrichment level considered to be HEU.
Calls for direct Iran- US talks
The discovery of HEU comes as the UN Secretary General Kofi Annan called on the United States to talk directly to Iran about its controversial nuclear program, saying that Tehran will not negotiate seriously if Washington is not involved.
"As long as the Iranians have a sense that they are negotiating with the Europeans ad referendum (needing referral for a final decision), and what they discuss with them will have to be checked with the Americans, and then come back again to them, I am not sure they will put everything on the table," Mr Annan told reporters.
But US State Department spokesman Sean McCormack has rejected Mr Annan's appeal, saying the Iranian government was interested only in “delaying and stalling".
Mr McCormack adding that Washington was comfortable backing European and Russian efforts to negotiate, "The problems that Iran has right now are with the rest of the world, not just between the United States and Iran… We believe that we are following the right diplomatic process now. We have been in support of the EU-3, we have been in support of the Russian government in their direct negotiations with the Iranian government."