A fire that broke out at Iran's Natanz nuclear facility on Thursday caused significant damage, the spokesman for the Atomic Energy Organization of Iran was quoted as saying on Sunday by the official IRNA news agency.
Spokesman Behrouz Kamalvandi said the fire could slow down Tehran's development and production of advanced centrifuges in the medium term, and that Iran would replace the damaged building with a bigger one that had more advanced equipment.
The New York Times quoted a Middle Eastern intelligence official Monday saying Israel had planted a bomb at the site. Israel's Defense Minister Benny Gantz said on Sunday that Israel was not "necessarily" behind every mysterious incident in Iran.
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The Natanz Fuel Enrichment Plant is one of several Iranian facilities monitored by inspectors of the International Atomic Energy Agency, the UN nuclear watchdog.
Iran's top security body said on Friday that the cause of the "incident" at the nuclear site had been determined, but "due to security considerations" it would be announced at a convenient time.
Iran's Atomic Energy Organisation initially reported an "incident" had occurred early on Thursday at Natanz, located in the desert in the central province of Isfahan.
Three Iranian officials who spoke to Reuters on condition of anonymity said they believed the fire was the result of a cyber attack, but did not cite any evidence.
One of the officials said the attack had targeted the centrifuge assembly building, referring to the delicate cylindrical machines that enrich uranium, and said Iran's enemies had carried out similar acts in the past.
Two of the officials said Israel could have been behind the Natanz incident, but offered no evidence.
An online video and messages released on Friday deepened the mystery around the incident. Released by a self-described group called the “Cheetahs of the Homeland,” it included language used by several exiled Iranian opposition organizations, as well as focused almost entirely on Iran's nuclear program, viewed by Israel as a danger to its very existence.
But filled with disparate messages, and released by a group unknown by Iran experts, the video has also sparked further suspicions that Natanz had again had faced sabotage by a foreign nation as it had during the Stuxnet computer virus outbreak believed to have been engineered by the U.S. and Israel.
The Associated Press contributed to this report.