Experts from the UN atomic watchdog are back in Iran to resume inspections of the country's nuclear facilities.
The inspectors will visit the gas centrifuge facility in Natanz
The Iranians suspended the inspections earlier this month, because of a dispute with the International Atomic Energy Agency (IAEA).
The IAEA accused Tehran of keeping some nuclear activities secret.
The US says Iran is using its nuclear power programme to develop weapons, while Iran insists it is designed to meet its energy needs.
A team of two inspectors arrived in Tehran on Saturday and immediately began work, IAEA spokesperson Melissa Fleming said.
They are due to visit a gas centrifuge enrichment facility at Natanz and the Isfahan nuclear research centre.
The inspectors are looking for any evidence that Iran has been trying to develop nuclear weapons.
ElBaradei is due in Iran in April
IAEA chief Mohammed ElBaradei is expected to visit Iran next month.
On 13 March the Vienna-based agency issued a resolution rebuking Iran for failing to disclose certain aspects of its nuclear programme, as it is expected to do as a signatory to the Nuclear Non-proliferation Treaty.
The US has called on Iran to suspend all uranium-related activity.
The IAEA has a June deadline to present a judgment on Tehran's nuclear activities.