The UN nuclear watchdog says Iran has agreed to a plan aimed at clearing up questions about its controversial nuclear activities.
UN inspectors can visit the Arak plant under the agreement
The IAEA says the development is "significant", but adds that for the plan to work, it is essential to get full and active co-operation from Iran.
It also says Iran is continuing its enrichment programme, but at a slower pace than before, despite UN sanctions.
Western powers fear Iran could try to make nuclear arms, which Tehran denies.
They have warned Iran is playing for time and should halt its programme immediately to avoid further UN sanctions.
The UN Security Council has already imposed two rounds of sanctions against Iran over the nuclear row.
In a confidential report, a copy of which was obtained by the BBC, the International Atomic Energy Agency (IAEA) said the work plan it had agreed with Iran to clear up key questions about its past nuclear activities was a "significant step forward".
But it added: "Once Iran's past nuclear programme has been clarified, Iran would need to continue to build confidence about the scope and nature of its present and future nuclear programme."
It said it was essential for Iran to stick to the agreed timeline.
The report said some issues such as Iran's plutonium experiments had been resolved and that it was hoping to have answers to other questions in the next few months.
The IAEA's work plan with Iran has been sharply criticised by a number of Western diplomats, says the BBC's Bethany Bell in Vienna, where the agency is based.
They accuse Iran of playing for time, delaying the imposition of further UN sanctions while increasing its nuclear capabilities.
And they have expressed concern that Iran is still enriching uranium in defiance of the Security Council.