The head of the UN nuclear agency arrived in Tehran tasked with the difficult mission of getting Iran to agree to new probes of some of the country's nuclear sites.
International Atomic Energy Agency (IAEA) director-general Yukiya Amano and his two aides were rapidly whisked away after landing at Tehran airport.
It is Amano's first visit to Iran since he became head of the IAEA in 2009.
The IAEA delegation is expected to meet with Iran's top nuclear negotiator, Saeed Jalili, Iran's foreign minister Ali Akbar Salehi and other officials.
The visit comes ahead of a key nuclear meeting in Baghdad between Tehran and a group of leading world powers on 23 May.
Amano's task will be to get the Islamic republic to agree to the IAEA investigating Iranian sites such as the Parchin military complex, where suspicious activity has been reported, giving rise to fears that Iran could use its uranium enrichment programme to build a nuclear bomb.
Iran has dismissed such fears as unfounded and insists its nuclear programme is for peaceful domestic purposes only.
During the last round of talks in January and February this year, one of the IAEA's key demands was to visit the Parchin site, but it was refused by Iranian authorities
Iranian foreign minister Salehi welcomed Amano's visit, but warned the IAEA team will not be inspecting any of the country's nuclear facilities.
His comments came as both Iran and the IAEA said they had made progress in a previous round of talks last week.
Remaining cautious ahead of the new talks, Amano said:"Nothing is certain in life, in diplomacy," he told reporters before departing from the airport in Vienna. "But there has been good progress.
"I really think this is the right time to reach an agreement," he added.
Amano's one-day trip is expected to set the mood for next week's meeting in Baghdad between Iran and the United States, Russia, China, Britain, France and Germany.
The six world powers will attempt to persuade Iran to curb its nuclear programme, while Iran is expected to seek prevent additional US and EU sanctions on its oil exports.