Iran will give inspectors access to records and equipment from two of its nuclear sites, the head of the UN's atomic agency, the IAEA, has said.
Iran says the Arak plant will make radio isotopes for medical use
Mohamed ElBaradei said he hoped Iran's move would begin a series of measures that would clear suspicions over its nuclear programme.
The IAEA has however rejected an Iranian request for help in building a heavy-water nuclear reactor at Arak.
The US fears Iran could use the reactor to make fuel for a nuclear weapon.
Plutonium released as a by-product in a heavy water reactor can serve as a substitute for highly-enriched uranium in constructing a nuclear device.
Iran has also repeatedly rejected demands to halt its uranium enrichment work.
The UN Security Council remains deadlocked over Iran's announcement that it will not suspend uranium enrichment as a precondition for talks over its nuclear programme.
Iran has dismissed US suspicions that it is building a bomb and insists its nuclear work does not have a military aspect.
According to Mr ElBaradei, Iran has agreed to let International Atomic Energy Agency (IAEA) inspectors take environmental samples from equipment at a former military site at Lavizan.
Iran has also said it will give the UN access to records from a uranium enrichment plant in Natanz.
Mr ElBaradei welcomed the moves but said Iran needed to show more transparency over its nuclear programme.
He told the AFP news agency that Iran needed to give "a full explanation of the development of its nuclear programme from start to finish".
Iran, he said, needed "to openly corroborate this explanation with evidence, including records and access to relevant locations and individuals involved".
According to the BBC's Pam O'Toole, Iran's latest offer may be trying to show it is co-operating with the IAEA - but western countries will probably argue more needs to be done if it is to avoid the threat of sanctions.
Diplomats meeting at IAEA headquarters in Vienna, Austria, rejected Iran's request for help in building the heavy water reactor.
BBC correspondents say the IAEA has left open the possibility of reconsidering Iran's request in the future.
Iranian Foreign Minister Manouchehr Mottaki said work on the Arak reactor would continue, regardless of the IAEA decision.
"It is the duty of the IAEA to help. If they help, we will appreciate it. If not, we will do it on our own," he said.
Iran says the heavy-water plant at Arak will help it make radio isotopes for medical purposes.
The US fears the reactor at Arak, when complete, could produce enough plutonium to build one bomb every year.