The International Atomic Energy Agency says it is negotiating with Iran on a definition of suspension of its uranium-enrichment activities agreed to by Tehran last year after international pressure.
The IAEA wants Tehran to suspend more activities related to enriching uranium that could be used to make nuclear weapons.
An IAEA board resolution adopted last year said Iran's enrichment activities were cause for grave concern and that suspension this work was of key importance.
The atomic energy agency says its inspectors in Iran are monitoring a large facility at Natanz that was kept secret from the agency until last year.
The inspectors say Iran is not installing new centrifuges at the plant but that it is going ahead with assembly and honoring contracts. Centrifuge technology can be used to purify uranium for peaceful purposes - or to make a bomb.
An IAEA spokesman, Lothar Wedekind, says Iran revealed the extent of the Natanz plant last summer.
"They did inform us that over 100, of approximately 1,000 planned centrifuge casings, had already been installed at the [Natanz] pilot plant," he said. "The overall plan was to have 50,000 of these centrifuges."
Scientists say that to enrich uranium to weapons-grade standard, several-thousand centrifuges are necessary.
Washington says Iran's sophisticated nuclear program, concealed from the IAEA for almost two decades, has no sensible use for civilian purposes. But Tehran insists its enrichment program is only to generate electricity for its growing population.
The head of the IAEA, Mohamed ElBaradei, says both Libya and Iran bought nuclear technology on the international black market. He has described this sophisticated global network as clever and fantastic, comparing trading in illegal nuclear material to organized crime.
Mr. ElBaradei told the German magazine Der Spiegel in an interview that he feared dictators or terrorists could get hold of nuclear weapons.
Mr. ElBaradei is to make a report on his inspectors' work in Iran and Libya to the IAEA board in mid-February.