File photo: Bushehr nuclear power plant in southern Iran. Photograph:( AFP )
Iran's Atomic Energy Organisation published a photo showing a shed at ground level had been partly burned.
Iran's nuclear body said an accident took taken place at a warehouse in a nuclear complex without causing casualties or radioactive pollution.
The Natanz Fuel Enrichment Plant (FEP), Iran's main uranium enrichment site, is one of the several Iranian facilities monitored by the United Nations nuclear watchdog.
Iran's Atomic Energy Organisation later published a photo showing a shed at ground level had been partly burned.
"There is some damage to the shed which we are investigating. It was inactive and there was no radioactive material in it and there were no personnel," the organisation's spokesman, Behrouz Kamalvandi, told state TV.
"There has been no interruption in the work of the enrichment site and no damage to the site."
The governor of Natanz city, Ramazanali Ferdosi, said the incident caused a fire but he gave no further details about the cause, the Tasnim news agency reported. Experts from the Atomic Energy Organisation are investigating.
Some experts did not rule out the possibility of sabotage given the importance of the Natanz nuclear site.
"Considering that this so-called incident happened just a few days after the explosion near the Parchin military base, the possibility of a sabotage cannot be ruled out," a former Iranianian nuclear official told Reuters.
"Also Natanz enrichment facility has been targeted in the past by a computer virus," he said, referring to an attack in 2010 by the Stuxnet computer virus that damaged centrifuges and is widely believed to have been developed by the United States and Israel.
Last Friday an explosion occurred east of Tehran near a sensitive military complex which the authorities said was caused by a tank leak in a gas storage facility in a public area.
Western security services believe Tehran carried out tests relevant to nuclear bomb detonations more than a decade ago at the Parchin military and weapons development base. Iran has denied it carried out such tests.
Iran agreed to curb its nuclear program in exchange for the removal of most international sanctions in a deal reached between Tehran and six world powers in 2015.
But Tehran has gradually reduced its commitments to the accord since US President Donald Trump's administration withdrew from the agreement in 2018 and reimposed and intensified sanctions that have battered Iran's economy.
The deal only allows Iran to enrich uranium at Natanz facility, with just over 5,000 of first-generation IR-1 centrifuges.
Israel has backed Trump's "maximum pressure" policy on Tehran aimed at forcing it to agree a new deal that puts stricter limits on its nuclear work, curbs its ballistic missile program and ends its regional proxy wars.
Iran says it will not negotiate as long as sanctions remain in place.
The International Atomic Energy Agency, which is policing the nuclear deal and carries out inspections in Iran said it was aware of reports about Natanz.
"We currently anticipate no impact on the IAEA's safeguards verification activities," it said in a statement.