UN nuclear inspectors arrived in Iran Saturday to visit the still-unfinished Arak heavy water plant for the first time in more than two years, the ISNA news agency reported.
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.
The Arak heavy water plant had been a sticking point in the negotiations because, once completed, it would produce plutonium as a by-product, potentially giving Tehran a second route to a nuclear weapons capability.
The two inspectors from the International Atomic Energy Agency will begin their work on Sunday at the site, 240 kilometres (150 miles) southwest of the capital, but ISNA did not say how long they would be there.
Iran's atomic energy organisation said it had provided the IAEA with "required information on ongoing research" about its new generation of centrifuges -- machines used to enrich uranium by spinning it at supersonic speed.
Spokesman Behrouz Kamalvand said the information was supplied "on time and within the framework of safeguard agreements," the official IRNA news agency reported.
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The inspection of Arak comes within the framework of a mid-November agreement between Iran and the IAEA that also allows the nuclear watchdog access to a uranium mine in Gachin, in the country's south.
The IAEA has visited Arak before, but says it has not received any new design details since 2006. Inspectors last visited the heavy water plant in August 2011.
Under the deal reached last month in Geneva with the P5+1 group of world powers -- the United States, Britain, France, China, Russia and Germany -- Iran agreed not to commission the reactor or transfer fuel or heavy water to the site for a period of six months.
However, construction delays have effectively made that a moot point, as the authorities do not expect to complete work on the plant before the end of 2014.
The accord reached with the P5+1 is seen as an interim agreement to give the two sides more time to reach a comprehensive deal on Tehran's nuclear programme.
Western nations and Israel have long suspected Iran of pursuing a nuclear weapons capability alongside its civilian programme, charges denied by Tehran.