Olli Heinonen, deputy director general of the International Atomic Energy Agency, is meeting with the Islamic Republic's top security and nuclear officials behind closed doors.
As was reported previously, he will address Iran's "purported nuclear research."
The IAEA said in a report released in late February that Iran had become more transparent on its nuclear program, but had failed to fully answer allegations relating to nuclear weapons development.
The international community has demanded that Tehran halt uranium enrichment, used both in electricity generation and nuclear weapons production. Iran insists on its right to civilian nuclear energy, and has defied three sets of United Nations sanctions over its nuclear program.
In a sign of defiance Iran said earlier this month it had started to install another 6,000 uranium enrichment centrifuges at its underground facility in Natanz in addition to its 3,000. The country also announced tests of advanced enrichment centrifuges, along with plans to build a second uranium processing plant by next March.
The country's nuclear ambitions have fueled tensions with Washington, with U.S. President George Bush refusing late last year to rule out military action against Tehran.
Russia and China, which both have strong business interests in Iran, blocked stronger measures against the country using their vetoes at the UN Security Council.