Iran acknowledged Monday that additional traces of weapons-grade uranium have been found on its soil but argued they came from abroad -- a claim U.N. and other experts said cannot be discounted.

The United States and its allies accuse Tehran of running a secret nuclear weapons program, and Iran's acknowledgment was expected to strengthen those arguments. Over the weekend, President Bush and Russian President Vladimir Putin (search) urged Iran to abandon its suspected nuclear-weapons programs.

Iran is facing an Oct. 31 deadline to bare its nuclear secrets set by the International Atomic Energy Agency (search) board of governors. If the board rules at its Nov. 20 meeting that Tehran has violated the Nuclear Nonproliferation Treaty banning the spread of nuclear arms, the Security Council could impose diplomatic or economic sanctions.

The IAEA is sending a team to Iran for negotiations Thursday ahead of what the agency hopes will be a new round of inspections starting Friday.

Iran insists it will not stop uranium enrichment and that it has a right to a peaceful nuclear program, as allowed under the Nuclear Nonproliferation Treaty.

Reacting to leaked reports last week, Ali Akbar Salehi, Iran's representative to the IAEA, told state-run Tehran television Monday that traces of highly enriched uranium had been found at the Kalay-e Electric Co., just west of Tehran.

But he ruled out that what was found there and at another facility at Natanz, was locally produced, saying the traces came in on contaminated equipment that was bought abroad.

That argument first surfaced in a report presented to the board meeting that set the October deadline, detailing the Natanz find and other activities feeding suspicions about Iran's nuclear intentions.

While expressing concern, IAEA Director General Mohamed ElBaradei (search), the author of the report, indicated then he could not dismiss Iran's version of events, suggesting it was one of "a number of possible scenarios."

Others concurred Monday.

"It is certainly not being dismissed by (IAEA) experts as being beyond the pale," said a diplomat familiar with the Iran issue, who spoke on condition of anonymity.

Iran has refused to say where it bought the contaminated equipment, but diplomats have told The Associated Press the most likely country of origin was Pakistan. Former IAEA inspector David Albright said that could mean Iran was telling the truth.

The isotopes found in the traces of weapons-grade uranium on centrifuges at Natanz this year do "not have the signature of Europe," he said by telephone from Washington. "The finger points at Pakistan."

"There is not enough information to discount what Iran says," said Albright, now president of the Institute for Science and International Security.

Still, he said, it was also possible that Iran clandestinely enriched small quantities of uranium to weapons-grade quality. That would point to attempts to develop a military nuclear program.

Christopher Paine, a nonproliferation expert at the Washington-based National Resources Defense Council, said Iran's argument was "plausible."

But Thomas Cochran, with the same think-tank, said it was unlikely that Iran would not have checked any used equipment for contamination.

He said establishing the source of the equipment was key to checking on Iran's assertion.

"Then, you should be able to go back to those countries, and you should be able to go to the facilities and confirm" whether isotope traces there matched those found in Iran, he said.

In a bid to show Iran's good intentions, Foreign Minister Kamal Kharrazi said Monday outside the United Nations that the country was prepared to undergo more severe inspections because it had "nothing to hide."

Foreign diplomats said last week that IAEA inspectors found minute quantities of weapons-grade uranium at the Kalay-e Electric Co. Earlier this year, U.N. inspectors found weapons-grade highly enriched uranium particles at a plant in Natanz that is supposed to produce only a lower grade for energy purposes.

European Union foreign ministers urged Tehran to abandon its suspected nuclear arms program. The ministers are expected to urge Iran to sign a new IAEA protocol to allow for unfettered inspections of nuclear sites.

Kharrazi said his country was willing to do so, "but we want to make sure that additional protocol is ... going to be enough," to ease pressure on his country.

Egyptian Foreign Minister Ahmed Maher, who met with Kharrazi on Monday, said the Iranians "showed flexibility. They hope -- as he mentioned to us -- that they will be met with flexibility from the other side."