Four UN nuclear inspectors arrived in Iran early on Sunday to inspect the country's second uranium enrichment facility.
The delegation from the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), would spend two or three days in Iran, starting its inspections on Sunday.
Iran announced in September that it was building a new nuclear fuel enrichment plant near the holy city of Qom, some 100 km south of Tehran.
The announcement has fuelled fresh concerns that Iran could conduct covert uranium enrichment activities, and the IAEA demanded that Tehran provide detailed information and access to the new nuclear facility as soon as possible.
At a meeting between Iranian officials and Iran-Six envoys from the US, Russia, China, Britain, France and Germany in Geneva on October 1, Iran's chief nuclear negotiator Saeed Jalili promised that Tehran would soon open the facility near Qom to UN inspectors.
The inspection comes three days after talks in the Austrian capital, Vienna, on the supply of nuclear fuel for a research reactor in Tehran. The Vienna talks, aimed at easing international suspicions of a covert Iranian atomic weapons program, involved Iran, the UN, the US, Russia and France.
Russia, France and the US Friday formally approved a package of IAEA, which involves Iran shipping 1.2 tonnes of low enriched uranium to Russia, where it would be enriched and sent on to France for processing into fuel rods.
The complex arrangement is necessary because only France produces fuel rods that will fit the Iranian research reactor, built in Tehran by Argentinean experts using French technology.
Iran asked Friday for more time to assess the deal, as some senior politicians have spoken out against the plan.
Iran is under three sets of UN Security Council sanctions over its refusal to halt uranium enrichment.