Melissa Fleming, a spokeswoman for the International Atomic Energy Agency, said the invitation was issued by Ali Larijani, Tehran's chief nuclear negotiator, in talks June 24 with IAEA head Muhammad el-Baradei.
She said the IAEA intended to send a team "as early as practicable."
The United States, Britain, Russia, France, Germany, and China are discussing a third round of Security Council penalties.
Iran says its campaign to enrich uranium for nuclear fuel is solely designed to generate electricity.
But suspicions persist in the West that Iran is secretly trying to build atomic bombs, since Tehran hid sensitive nuclear research from the IAEA for almost 20 years and has stonewalled investigations since.
(AP, Reuters, AFP)