Iran's Natanz nuclear site suffered a problem today involving its electrical distribution grid just hours after starting up new advanced centrifuges that more quickly enrich uranium, state TV reported.
It was the latest incident to strike one of Tehran's most secure sites amid negotiations over the tattered atomic accord with world powers.
State TV quoted Behrouz Kamalvandi, a spokesman for Iran's civilian nuclear programme, announcing the incident.
"Kamalvandi said fortunately the incident has not caused any human damage or contamination," a state TV anchorwoman said. "The cause of the incident is under investigation."
The word state television used in its report attributed to Kamalvandi in Farsi also can be used for "accident".
The Atomic Energy Organisation of Iran, the civilian arm of its nuclear programme, later published a statement using the same wording as the TV report, without elaborating.
Natanz, a facility earlier targeted by the Stuxnet computer virus, was largely built underground to withstand enemy airstrikes. It became a flashpoint for Western fears about Iran's nuclear programme in 2002, when satellite photos showed Iran building its underground centrifuges facility at the site, some 200km south of the capital, Tehran.
Natanz suffered a mysterious explosion at its advanced centrifuge assembly plant in July that authorities later described as sabotage. Iran now is rebuilding that facility deep inside a nearby mountain.
Israel, Iran's regional archenemy, has been suspected of carrying out an attack there, as well as launching other assaults, as world powers now negotiate with Tehran in Vienna over its nuclear deal.
Iran also blamed Israel for the killing of a scientist who began the country's military nuclear programme decades earlier. Israel has not claimed any of the attacks, though Prime Minister Benjamin Netanyahu repeatedly has described Iran as the major threat faced by his country in recent weeks.
Israeli officials could not be immediately reached for comment. US Defence Secretary Lloyd Austin landed in Israel today for talks with Netanyahu and Israeli Defense Minister Benny Gantz.
Natanz today hosts the country's main uranium enrichment facility. In its long underground halls, centrifuges rapidly spin uranium hexafluoride gas to enrich uranium.
Iran yesterday announced it had launched a chain of 164 IR-6 centrifuges at the plant. Officials also began testing the IR-9 centrifuge, which they say will enrich uranium 50 times faster than Iran's first-generation centrifuges, the IR-1. The nuclear deal limited Iran to using only IR-1s for enrichment.
Since then-President Donald Trump's withdrawal from the Iran nuclear deal in 2018, Tehran has abandoned all the limits of its uranium stockpile. It now enriches up to 20 per cent purity, a technical step away from weapons-grade levels of 90 per cent. Iran maintains its atomic programme is for peaceful purposes, but fears about Tehran having the ability to make a bomb saw world powers reach the deal with the Islamic Republic in 2015.
The deal lifted economic sanctions on Iran in exchange for it limiting its programme and allowing inspectors from the International Atomic Energy Agency to keep a close watch on its work.
On Tuesday, an Iranian cargo ship said to serve as a floating base for Iran's paramilitary Revolutionary Guard forces off the coast of Yemen was struck by an explosion, likely from a limpet mine. Iran has blamed Israel for the blast. That attack escalated a long-running shadow war in Mideast waterways targeting shipping in the region.