The admission came ahead of key negotiations this week with the U.N. International Atomic Energy Agency over inspections of Iran's nuclear program. The United States accuses Iran of seeking to develop nuclear weapons, while Tehran insists its program is peaceful.
The find of nuclear-grade enriched uranium came at the Kalay-e Electric Co., just west of Tehran, said Ali Akbar Salehi, Iran's representative to the IAEA.
Salehi, speaking on Tehran television, ruled out that the enriched uranium found at the site and another facility at Natanz was produced in Iran. Tehran maintains that traces of the new enriched material were imported on equipment purchased from abroad.
The United States, however, has pointed the discoveries as evidence that Iran is secretly developing nuclear weapons, and Russia and the European Union have joined in pressing Iran to come clean. The IAEA has set an Oct. 31 deadline for Tehran to consent to unfettered inspections of its facilities.
Also Monday, Iran's government spokesman Abdollah Ramezanzadeh said Iran's right to have a peaceful nuclear program will not be compromised.
"We don't accept any restrictions on the peaceful use of nuclear energy," he told reporters. "Peaceful use of nuclear energy is the right of the Iranian nation and we won't compromise on this."
Ramezanzadeh said Tehran has cooperated fully with the IAEA and future cooperation will depend on upcoming talks with agency inspectors.
Foreign diplomats said last week that IAEA inspectors found minute quantities of weapons-grade uranium at Kalay-e, where Iran reportedly tested centrifuges used to enrich uranium. Earlier this year, U.N. inspectors found highly enriched uranium particles at a plant in Natanz that is supposed to produce only a lower grade for energy purposes.
Iran previously blamed contaminated equipment for the readings at Natanz.
Salehi said Iranian and IAEA officials were surprised that enriched uranium had been found at the two sites.
It was "unexpected…because it needs a lot of centrifuges to work for a long time to enrich uranium," he told the TV station. "The IAEA and we know that there has been no such level of activity in Iran."
In Vienna, IAEA spokesman Mark Gwozdecky would not comment on the Iranian acknowledgement. He said a team of senior agency inspectors will begin meetings in Tehran on Thursday and they would be joined by "technical experts" on Friday ready to begin a new round of inspections.
A diplomat familiar with the Iran nuclear issue, speaking on condition of anonymity, said there was no way of telling from the samples available to the IAEA whether the Iranian explanation was true.
In recent weeks, agency inspectors had been given access to sites not covered in previous agreements with Tehran, although the IAEA had complained of delays that in some cases appeared to have served to give authorities a chance to cover their tracks.
In August, Iran allowed inspectors to visit Kalay-e after they were turned away two months before when they came to take environmental samples.
On Sunday, Iran's foreign minister said his country is willing to cooperate with the U.N. nuclear agency.
"We are trying and we are determined to cooperate" with the IAEA, Foreign Minister Kamal Kharrazi said in an interview broadcast Sunday on ABC's "This Week."
On Saturday at Camp David, President Bush and Russian President Vladimir Putin urged Iran to abandon its suspected nuclear-weapons programs, although Putin refused to stop plans to build a power plant in Iran.
Russia is a major player in the development of Iran's nuclear program, a fact that has being an issue of concern with the United States.
Two weeks ago, the State Department imposed sanctions on a state-owned Russian firm for selling arms to Iran, Agence France Presse reported. The sanctions were then waived, as the U.S. does in cases where a friendly government is involved.