The four-member team of inspectors from the International Atomic Energy Agency (IAEA) arrived in Tehran on Sunday and later headed to the plant being built inside a mountain near the Shiite holy city of Qom.
Iranian officials have previously said the facility is created adjacent to a military base south of the capital and hence is heavily guarded in case of any air strike by archfoes US or Israel.
Ali Shirzadian, the spokesman of Iran's atomic body, confirmed to AFP that the UN team had inspected the plant on Sunday.
The Mehr news agency meanwhile reported that the UN team will make several visits to the plant during their three-day stay in Iran.
The inspectors are checking the plant to verify whether the facility was designed for peaceful nuclear purposes.
The disclosure by Tehran of its existence on September 21 triggered widespread outrage in the West, which suspects Iran is enriching uranium with an ultimate goal of using it to make atomic weapons. Chronology: Developments in standoff
Tehran strongly denies the charge.
Iran has already been enriching uranium — the most controversial aspect of its nuclear project — for several years at another plant in the central city of Natanz, in defiance of three sets of UN sanctions.
Enriched uranium produces fuel for civilian reactors, but in highly extended form can also make the fissile core of an atomic bomb.
During their inspection the experts are expected to compare the information of the plant provided by Iran with what they actually find at the facility, Mehr reported on Saturday quoting a Vienna-based official. The IAEA is headquartered in Vienna.
The inspectors are also expected to take environmental samples from around the plant to analyse if any radioactive material has been moved into the facility.
Iranian officials have previously said that no radioactive materials have been put inside the plant as it is still under construction.
