Iran agreed last year to halt uranium enrichment and allow wider inspections of its nuclear facilities.
But diplomats tell Agence France-Presse that while Tehran appears to have stopped enriching uranium, it is still producing the centrifuges that make that enrichment possible, in case Iran wants to resume the process later on.
"That is something we don't view favorably," AFP quoted one diplomat as saying.
The U.N.'s nuclear watchdog, the International Atomic Energy Agency, did not comment.
Iran insists all its nuclear research is for civilian purposes. When traces of enriched uranium were found on Iranian equipment last year, Tehran claimed the material was left over from previous use in other countries.
The United States accuses the Iranian government of trying to develop weapons. Announcing aid to victims of an earthquake in Iran earlier this month, President Bush said Iran "must abandon their nuclear weapons program."
The U.S. doubts Iran, which is rich in oil, needs nuclear energy for civilian purposes.
In a report last year, the IAEA found "no evidence" Iran is trying to make nuclear weapons, but cannot rule out the possibility because Tehran previously hid parts of its program.
In November, the agency censured Iran for 18 years of secrecy about its nuclear activities. IAEA head Mohamed ElBaradei said the measure sent an "ominous message that failures in the future will not be tolerated."
The resolution warns against "further serious Iranian failures," saying that could lead the board to consider actions allowed by its statute — shorthand for possible referral to the Security Council.
It is not clear if the production of centrifuges would constitute a failure in the eyes of the IAEA.
After the Sept. 11 attacks, the Bush administration labeled Iran part of an "axis of evil" along with Iraq and North Korea, which Washington also suspects of developing weapons of mass destruction.
The United States has asserted for years that Iran was bent on obtaining nuclear arms.
In 1998, a commission on the ballistic missile threat — headed by now Defense Secretary Donald Rumsfeld — found that: "If Iran were to accumulate enough fissile material from foreign sources, it might be able to develop a nuclear weapon in only one to three years."