A problem with electrical power caused an incident in Iran's Natanz underground nuclear facility, Iranian Press TV reported, a day after Tehran launched new advanced uranium enrichment centrifuges in the site.
"The incident caused no casualties or pollution," Iran's Atomic Energy Organization spokesman Behrouz Kamalvandi said, adding that "electricity was affected at the Natanz facility".
Last year, a fire broke out at the Natanz nuclear facility, which the government said was an attempt to sabotage Iran's nuclear programme.
The underground Natanz site is the centrepiece of Iran's uranium enrichment programme and monitored by inspectors of the International Atomic Energy Agency, the UN nuclear watchdog.
Tehran and Washington have been trying to revive Iran's 2015 nuclear deal with major powers that former US President Donald Trump abandoned three years ago. Trump reimposed sanctions that were lifted on the Islamic Republic, and brought in many more.
In reaction to the US sanctions, Iran breached many restrictions imposed by the accord. The two nations laid out tough stances at indirect talks in Vienna last week on how to bring both back into full compliance with the accord.