U.N. inspectors visited an Iranian plant on Sunday linked to a planned heavy-water reactor that could yield nuclear bomb fuel, taking up an initial offer by Tehran to open its disputed nuclear programme up to greater scrutiny.
The increased transparency is one of the various spin-offs from an interim deal that Iran struck with six world powers last month to curb its nuclear programme in return for some easing of sanctions.
It was the first time in more than two years that the International Atomic Energy Agency (IAEA) had been allowed to go to the Arak heavy water production plant, which is designed to supply a research reactor under construction nearby.
Iran's heavy water work is of great concern for the West because it could in theory be used in the process of making a nuclear bomb, although Tehran says the programme is for peaceful purposes.
Two inspectors arrived in Tehran on Saturday and met experts from Iran's own atomic energy agency before travelling to Arak in the evening, Iran's ISNA news agency reported.
"The inspection is under way and will be finished this afternoon, and they (the inspectors) will return to Tehran," said Behrouz Kamalvandi, spokesman for the Iranian atomic energy agency. "The inspectors will go back to Vienna tonight."
Officials from Iran and the United States, France, Germany, Britain, China and Russia are to meet on Dec. 9-10 in Vienna, where the IAEA is based, for expert-level talks on implementing the interim accord.
Kamalvandi told ISNA there was a "strong possibility" that the timing of another inspection, of the Gachin uranium mine in southern Iran, would be discussed. The IAEA says it wants to visit Gachin to get a better understanding of Iran's nuclear programme.
U.S. officials have said Washington might press Iran to dismantle part of the unfinished Arak nuclear reactor, but Kamalvandi said Tehran would not entertain such a demand.
"We won't accept getting into discussions about such issues. Iranian officials have repeated their stance over and over again: Iran's nuclear rights are non-negotiable."