EU spokesman Peter Stano said the reported incident "could have been an act of sabotage" but insisted that there had been no official attribution over who was responsible.

"We still need to clarify the facts in detail as quickly as possible," he said, adding: "We reject any attempts to undermine or weaken diplomatic efforts on the nuclear agreement."

The Natanz site near Tehran was hit by a power outage on Sunday that Iran labelled "terrorism", a day after officials announced the country had started up advanced uranium enrichment centrifuges.

Negotiators in Vienna are trying to find a way to return the United States to the 2015 pact with world powers aimed at curbing Iran's nuclear ambitions, after former president Donald Trump withdrew.

The EU's mediator on Friday described the talks as "constructive", as those involved look to persuade Washington to drop sanctions reimposed by Trump and for Tehran to roll back breaches of the deal.

New US President Joe Biden has said he wants to revive the agreement, which places limits on the Islamic republic's nuclear programme in return for relief from biting economic sanctions.

Israel strongly opposes the deal and has vowed to disrupt any efforts by Iran to build an atomic bomb -- a goal Tehran has always strongly denied pursuing.