No reason was given for Tero Varjoranta's sudden departure, which comes days after US President Donald Trump took the US out of the deal between Iran and world powers over its nuclear programme.

The IAEA conducts inspections in Iran to verify compliance with the terms of the deal.

"The Agency's safeguards activities will continue to be carried out in a highly professional manner," the spokesperson said.

Varjoranta, who had been in post since late 2013, has been replaced on a temporary basis by Massimo Aparo, the acting director of the Agency's Office for Verification in Iran.

Varjoranta is Finnish and had previously served as head of Finland's Radiation and Nuclear Safety Authority.

A permanent replacement will be appointed as soon as possible, the IAEA spokesperson said.

- Fears for deal's future -

US officials have said that despite the US's withdrawal from the Iran accord, they want the IAEA's inspections in Iran to continue.

"The United States will continue to support robust implementation of IAEA inspections in Iran to the full extent of the IAEA's authority," an official at the US mission to the IAEA told AFP on Friday.

Known officially as the Joint Comprehensive Plan of Action (JCPOA), the deal between Tehran and major world powers forces Iran to open any site to the IAEA within 24 days at most.

Since the accord was reached in 2015, the IAEA has carried out hundreds of inspections inside Iran.

The IAEA says that thanks to access afforded under the accord, inspection work has doubled since 2013, with its inspectors now spending 3,000 man-days per year on the ground in Iran last year.

The agency also points to the some 2,000 tamper-proof seals attached to nuclear material and equipment and to the "hundreds of thousands of images captured daily by our sophisticated surveillance cameras", the number of which has almost doubled since 2013.

Since the JCPOA came into force, the IAEA has issued regular quarterly reports confirming that Iran is adhering to its "nuclear-related commitments" under the deal.

The other parties to the deal, including Russia, China and European powers, say they want to keep it alive despite Trump's move.

However, some analysts fear a collapse of the deal now that the US has withdrawn and re-imposed sanctions on Iran, which may lead to Iran ending co-operation with the IAEA's inspections.