The head of the UN's nuclear watchdog says Iran has agreed to consider tougher inspections of its controversial nuclear energy programme.
Iran says it has nothing to hide from inspectors
Mohammed ElBaradei said agreement had been reached to send a team of experts to Iran "to discuss with Iran the areas that Iran needs clarification on".
The director of the International Atomic Energy Agency (IAEA) was speaking to reporters at the end of what he called "useful and constructive" talks in Tehran.
His visit was aimed at pressing for tighter inspections of Tehran's nuclear facilities.
Iran has so far resisted calls to sign an Additional Protocol to the Nuclear Non-Proliferation Treaty (NPT).
This would allow IAEA experts to conduct more rigorous visits to Iran's nuclear sites at short notice.
The BBC's Jim Muir in Tehran says the visit was not perhaps the breakthrough Mr ElBaradei was hoping for but, nevertheless, it signalled some progress.
"I hope that once these issues have been clarified that Iran will be in a position to sign the protocol," Mr ElBaradei said after his meetings with Iranian leaders.
Allows for inspections at short notice
IAEA can take environmental samples at any location
Two top nuclear experts will stay in Iran to be joined by other IAEA officials next week when they will address various concerns with Iran.
The head of Iran's own nuclear agency, Gholamreza Aghazadeh, said his agency would prepare a report on the Additional Protocol after which the Iranian Government would "take the necessary decision."
Iran - which denies charges by the United States and Israel that it is trying to develop nuclear weapons - has insisted that a ban on the country's access to nuclear technology will have to be lifted before it can agree to sign the protocol.
Russia and France have urged Tehran to sign it, saying that it would eliminate concerns over its nuclear programme.
Mr ElBaradei's visit came just days after Israel voiced grave concerns over a final test of an Iranian missile capable of hitting Israeli territory.
In June, Mr ElBaradei said that so far Tehran had failed to report some of its nuclear activities.
On a visit in February, Mr ElBaradei toured a nuclear plant under construction at Natanz, 320 kilometres (200 miles) south of Tehran.
The site is crucial, because it is where Iran is developing a series of centrifuges, which could be used to produce enriched uranium - the material used for making a nuclear bomb.
Iran has repeatedly rejected the IAEA'S accusations as "politically motivated and politically charged", saying the aim of its programme is to generate electricity.
On Tuesday, an exile Iranian opposition group - National Council of Resistance of Iran (CNRI) - presented details of what it said was a new site where Iran planned to develop uranium enrichment technology.
Satellite image of Iran's nuclear power reactor in Bushehr
At a press conference in Washington, the group said the site was located at Kolahdouz, about 15 kilometres (10 miles) from Tehran.
CNRI's representative Alireza Jafarzadeh said that the Koahdouz facility was hidden in an industrial complex and was testing centrifuges for uranium enrichment.
The group also said that Iran was building another site at Ardekan, about 30 km (20miles) from Tehran, which was scheduled to be completed within two years.