Nuclear inspectors from the United Nations have visited a site where Iran has been accused of working on a nuclear weapons programme.
The head of the International Atomic Energy Agency, Mohamed ElBaradei, said his team had been given prompt access to the Lavizan site in northern Tehran.
Mr ElBaradei said the inspectors had taken some samples, and would visit further sites in Iran on Tuesday.
The US has accused of secretly trying to develop nuclear weapons.
Iran denies that Lavizan was ever a nuclear site, and insists it has no banned weapons programme.
The IAEA takes samples to test for traces of nuclear materials that might indicate signs of undeclared activities.
The IAEA found traces of enriched uranium at various Iranian sites last year - which Tehran says must have come in with equipment from abroad.