The existence of the formerly covert plant was officially revealed late last week after years of research by foreign intelligence agents.
Permission to visit the Qom site was granted as Iranian officials met in Geneva with representatives from the United States, Russia, Europe and China.
Iranian diplomats were offered upgraded economic and political ties in exchange for opening all of their country's nuclear program to international inspectors.
Iranian President Mahmoud Ahmadinejad announced his intention to allow inspection of the site when its existence became public knowledge. “We have no secrecy, we work within the framework of the IAEA,” he said at the time, referring to the UN's International Atomic Energy Agency.
European and United States leaders expressed satisfaction with the announcement, and said they expected oversight of the Qom plant to begin immediately.
French official Jacques Audibert said Iran must allow inspections within two weeks, and must provide “proof of a deep evolution in the management” of its nuclear program.
U.S. President Barack Obama made a similar demand, saying Iran had two weeks to grant “unfettered access” to international inspectors. “Our patience is not unlimited,” he warned.
Earlier in the day diplomats expressed cautious optimism over the Geneva talks, saying Iran had avoided an in-depth discussion of its nuclear program but had expressed willingness to meet again.
Foreign leaders later announced that Iran had also agreed to allow a foreign power to process its low-enriched uranium for use in a medical research center.