An "accident" took place at an Iranian nuclear facility on Sunday but caused no casualties or damage, the Fars news agency reported, citing the spokesman of Iran's nuclear agency.
Behrouz Kamalvandi said there had been "an accident in part of the electrical circuit of the (uranium) enrichment facility" at the Natanz complex, a day after Iran announced it had started up advanced uranium enrichment centrifuges at the site in a breach of its undertakings under a troubled 2015 nuclear deal.
There were "no casualties nor pollution", he said, adding that "the causes of the accident are under investigation and more details will be released later".
The accident follows an explosion at a factory for advanced centrifuges at Natanz last July.
The authorities blamed that incident on "sabotage" by "terrorists", but have not released the results of their investigation into it.
President Hassan Rouhani on Sunday inaugurated a replacement factory at Natanz as well as a cascade of centrifuges for enriching uranium and two test cascades, in a ceremony broadcast by state television.
The new equipment enables Iran to enrich uranium more quickly and in higher quantities, to levels that violate the 2015 nuclear deal.
The administration of then-US president Donald Trump withdrew from the multilateral accord in 2018 and re-imposed biting sanctions. Iran later responded by progressively rolling back its own commitments under the agreement.
Parties to the deal opened talks in Vienna on Tuesday with the aim of bringing the US back to the deal, bringing Iran back into compliance and lifting sanctions.
The accident took place prior to the arrival of US Defense Secretary Lloyd Austin in Israel Sunday on the first high-level visit from the new US administration, after talks restarted on reviving the 2015 Iran nuclear deal.
Austin's visit comes just days after Washington said it had offered "very serious" ideas on reviving the hobbled agreement that is staunchly opposed by Israel.
Austin's first scheduled stop on the two-day trip was a meeting with Israeli Defence Minister Benny Gantz.
Israel under hawkish Prime Minister Benjamin Netanyahu has been a fierce critic of the Iran nuclear deal, dating back to when it was being negotiated during Barack Obama's administration.
Netanyahu applauded when Trump withdrew from the deal and imposed sanctions on Tehran, which responded by stepping back from several of its commitments under the deal.
In the latest breach of its undertakings in the troubled agreement, Tehran announced on Saturday that it had started up advanced uranium enrichment centrifuges.