The head of the UN nuclear watchdog has called for swifter co-operation from Iran in resolving questions about its atomic programme, which the West fears will be used to produce atomic bombs.
"I discussed with (Iran's atomic energy chief) how we can work together to accelerate the pace of our co-operation to clarify all outstanding issues before my report in March," International Atomic Energy Agency chief Mohamed ElBaradei said on the first day of a visit to Tehran.
He said the talks - rare high-level diplomacy in Tehran to help defuse Iran's stalemate with world powers over its disputed nuclear ambitions - were "frank and friendly ... and I'm looking forward to an environment of accelerated co-operation".
"I asked Mr (Gholamreza) Aghazadeh to give us maximum transparency and provide assurances about all present nuclear activities," Mr ElBaradei told reporters.
"If we are able to clarify past and current activities, it will provide an atmosphere to overcome the standoff between the P5+1 and Iran," he added, referring to the six world powers who sponsored UN sanctions to curb Iran's nuclear programme.
Mr ElBaradei will meet Iranian Supreme Leader Ayatollah Ali Khamenei, President Mahmoud Ahmadinejad and top nuclear negotiator Saeed Jalili on Saturday, before returning to Vienna.
Aghazadeh said the IAEA could be assured of full co-operation and added: "Tomorrow during (ElBaradei's) meeting with the leader (Khamenei) there will be an extremely important exchange of information". He did not elaborate.
The Vienna-based IAEA has long sought to verify Iran's uranium enrichment programme is geared solely to producing civilian energy, not atomic bombs as Western powers suspect.
Mr Aghazadeh called the first talks "very good and comprehensive" and predicted Mr ElBaradei would be able to give a final report on its four-year-old Iran inquiry to the agency's 35-nation board of governors in March.
"Now we have entered a new phase and my suggestion is to create such an atmosphere that all issues can be resolved," he said. "It requires a political will and my suggestion to all parties is to use this opportunity for resolving the issues."
A diplomat close to the IAEA said before Mr ElBaradei's visit that an IAEA inquiry stonewalled by Iran for years until August had entered a final phase with Iran addressing US intelligence about past, covert attempts to "weaponise" atomic material.
Iran said in August it would answer outstanding questions in sequence about its nuclear past but an end-of-year target mooted by Mr ElBaradei for completing the process passed with the most sensitive issues still unresolved.