The UN nuclear agency and Tehran have held new talks on allegations of past Iranian weapons work and on additional safeguards to allay international concerns over its nuclear ambitions.
The day-long discussions with the Vienna-based International Atomic Energy Agency (IAEA) on Saturday will build on a framework deal agreed in November that required Tehran to take six practical steps by next Tuesday.
With completion of those measures - including a visit to the heavy water plant at the unfinished Arak reactor - talks on "more difficult things" are expected to begin, IAEA chief Yukiya Amano has said.
A team of IAEA experts led by chief inspector Tero Varjoranta arrived in Tehran late Friday to assess the implementation of those measures, Iranian Atomic Energy Organisation spokesman Behrouz Kamalvandi said.
That assessment will decide the scope of future co-operation, he said, expressing the hope that "the agency's doubts have been removed".
The experts met nuclear officials led by Iran's IAEA envoy Reza Najafi, Kamalvandi said in remarks reported by the official IRNA news agency.
The talks could be extended if there is major progress, media reports said.
The six-step deal was struck on November 11 after two years and nearly a dozen rounds of talks.
It is separate to the landmark nuclear agreement also reached in November with world powers that has placed temporary curbs on Iran's nuclear activities.
Implementation began on December 8 when IAEA inspectors visited Arak, where the small unfinished heavy water reactor has been hit by delays.
The site - which Iran insists is an integral part of its nuclear program for mainly research purposes - is of international concern because Tehran could theoretically extract weapons-grade plutonium from spent fuel if it also builds a reprocessing facility.
Iran's atomic chief Ali Akbar Salehi said this week the reactor could be modified to produce less plutonium to "allay the worries".
