Iran has reportedly halted its most sensitive nuclear activity under a ground-breaking deal with world powers.
A confidential United Nations atomic agency report obtained by the Reuters news agency also says Iran had begun diluting its stockpile of uranium enriched to the fissile concentration of 20% - a level that took it closer to the capability of producing fuel for an atom bomb.
The report by the International Atomic Energy Agency to member states said: "The Agency confirms that, as of 20 January 2014, Iran ... has ceased enriching uranium above 5 percent U-235 at the two cascades at the Pilot Fuel Enrichment Plant (PFEP) and four cascades at the Fordow Fuel Enrichment Plant (FFEP) previously used for this purpose."
It was referring to Iran's two enrichment plants, at Natanz and Fordow. Cascades are interlinked networks of centrifuge machines that enrich uranium. Iranian state television earlier said Iran had suspended 20% enrichment at Natanz and that inspectors were heading to Fordow.
The IAEA report also listed other measures Iran had agreed to under the interim accord with the six world powers - the United States, France, Britain, Germany, China and Russia.
Those included an undertaking that Iran would not build any more enrichment sites during the six-month agreement, a step meant to buy time for negotiations on a final settlement of Tehran's decade-old nuclear dispute with the six powers.
The actions pave the way for the easing of some Western sanctions.
Enriched uranium can have both military and civilian purposes. Iran denies Western allegations that it has been seeking to develop the capability to make nuclear bombs, saying it wants only civilian atomic energy.