The head of the UN nuclear watchdog, Mohamed ElBaradei, is visiting Iran to press for tighter inspections of Tehran's nuclear facilities.
Iran says it has nothing to hide from inspectors
Mr ElBaradei, who heads the International Atomic Energy Agency (IAEA), will renew his call for Iran to sign the so-called additional protocol to the Nuclear Non-Proliferation Treaty (NPT), allowing such inspections.
But Iran - which denies charges by the United States and Israel that it is trying to develop nuclear weapons - insists that a ban on the country's access to nuclear technology will have to be lifted before it can agree to such a move.
Russia and France have urged Tehran to sign the protocol, saying that it would eliminate concerns over its nuclear programme.
Allows for inspections at short notice
IAEA can take environmental samples at any location
Mr ElBaradei's visit comes just days after Israel voiced grave concerns over a final test of an Iranian missile capable of hitting Israeli territory.
Meanwhile, an exiled Iranian opposition group said on Tuesday that Iran had an undeclared site where it planned to perfect uranium enrichment technology for its suspected nuclear weapons programme.
Call for co-operation
Speaking at Frankfurt airport before departing for Tehran on Tuesday, Mr ElBaradei said that it was in Iran's best interests to accept more intrusive inspection regime.
"If they think this through, they will see it is very much in their interest to take the first step," Mr ElBaradei told Reuters news agency.
He also said that sustained co-operation from Iran could lead to a gradual lifting of the embargo by the Nuclear Suppliers Group (NSG).
In his June report, 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 was to generate electricity.
But last month, an Iranian representative at the IAEA's Vienna headquarters said the nuclear issue would be eventually resolved.
On Tuesday, the exiled Iranian opposition group - National Council of Resistance of Iran (CNRI) - presented 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.