Atomic experts from the UN nuclear watchdog were at Iran's Arak heavy water plant on Sunday to inspect it for the first time in more than two years, media reported.
"The IAEA experts started their visit to the Arak facility on Sunday morning," Fars news agency quoted Iran's atomic energy organisation spokesman Behrouz Kamalvandi as saying.
The visit comes just weeks after Iran clinched a landmark nuclear agreement with world powers under which it will freeze or curb some of its controversial nuclear activities in return for limited relief from crippling international sanctions.
Kamalvandi said the one-day inspection of the site 240 kilometres (150 miles) southwest of the capital would wrap up in the afternoon, and the International Atomic Energy Agency inspectors would leave for Vienna afterwards.
The inspection comes within the framework of a mid-November agreement that also allows IAEA access to a uranium mine in Gachin, in the country's south.
The IAEA regularly checks work on the Arak reactor, but says it has not received any new design details since 2006. And inspectors had not been to the heavy water plant at the facility since August 2011.
The Arak reactor is of concern to the international community because Tehran could theoretically extract weapons-grade plutonium from its spent fuel.