Coming a month before a UN deadline for Tehran to prove it has no secret atomic weapons programme, the latest discovery is sure to fuel the worldwide debate over Iran's suspected nuclear ambitions.
Bolstered by the finding, US President George Bush said on Thursday that Iran would face "universal condemnation" if it keeps pursuing a nuclear weapons programme.
Bush also told reporters that Iran's alleged pursuit of a nuclear weapon will be on the agenda for his talks on Friday and Saturday with Russian President Vladimir Putin.
Diplomats speaking on condition of anonymity told Reuters news agency that new traces of enriched uranium were found in environmental samples taken during inspections at the Kalaye Electric Company on the southern outskirts of Tehran.
A spokeswoman for the International Atomic Energy Agency (IAEA) refused to confirm or deny the claim, and said the agency was focussing on Iran's 31 October deadline and the departure of inspectors to Tehran on Sunday for a month of intensive inspections and talks.
Some months earlier, inspectors from the IAEA had found traces of enriched uranium at a plant at Natanz, some 250km south of the Iranian capital.
Iran blamed the find at Natanz on machinery which it says was contaminated with enriched uranium when it was purchased abroad on the black market in the 1980s.
But White House has been dismissive of Iran's explanation.
"These are part of a long-standing pattern of evasion and deception to disguise the true nature and purpose of Iran's nuclear activities," White House spokesman Scott McClellan said.
Some diplomats said the new finding could mean Iran has been covertly testing its enrichment machinery at Kalye.
Iran maintains its nuclear programme is for peaceful purposes.