An International Atomic Energy Agency team arrived in Iran early Sunday on a mission to clear up what it called "outstanding substantive issues" on Tehran's nuclear programme, the official IRNA news agency said.

The UN atomic watchdog's chief inspector, the Belgian Herman Nackaerts, is leading the IAEA delegation that is scheduled to hold talks with Iranian officials from later Sunday to Tuesday, the report added.

The delegation also included IAEA number two Rafael Grossi, IRNA said, adding the team "will probably visit the Fordo" enrichment site south of the capital Tehran.

Earlier this month, the IAEA said Iran had begun enriching uranium to 20-percent purity deep inside a mountain bunker at Fordo, taking it significantly closer to the 90-percent mark needed for a nuclear bomb.

IRNA provided no further detail, but diplomats in Vienna said the IAEA's senior legal official Peri Lynne Johnson, a US citizen, was to be in the delegation.

The visit comes after a damning report by the atomic watchdog agency in November raised suspicions that the Islamic republic had done work on developing nuclear weapons.

The report led to a substantial increase in pressure on Iran from the United States, the European Union and others.

With Iran vehemently denying that it wants nuclear weapons and dismissing the IAEA report as baseless, the watchdog's chief Yukiya Amano on Friday urged the Tehran to show "substantial cooperation" during the visit.

The Islamic republic, is already under four rounds of United Nations sanctions for refusing to stop enriching uranium until the IAEA is satisfied its programme is peaceful.