The fire broke out at a plant at the Natanz Nuclear Power Plant, 32 miles south of the capital Tehran. No people were physically affected but the material damage was severe, says Iranian Atomic Energy spokesman Behroz Kamalvandi, according to the state news agency Irna on Sunday.

"The incident could slow down the development and production of advanced centrifuges in the medium term ... Iran will replace the damaged building with a larger one that has more advanced equipment," says Kamalvandi.

Israel and several Western countries accuse Iran of trying to acquire nuclear weapons, and in 2010 Natanz was attacked by the Stuxnet computer mask - which is widely believed to have developed in the United States and Israel.

Advanced centrifuges

In 2015, Iran agreed with the P5 + 1 countries (the five permanent members of the UN Security Council and Germany) on restrictions in the country's nuclear program in exchange for sanctions relief, under the JCPOA agreement. In 2018, US President Donald Trump resigned from the agreement, prompting Iran to waive certain commitments the following year, including enrichment of uranium.

The JCPOA allowed Iran to have just over 5,000 first-generation centrifuges at the mainly underground facility in Natanz, which is monitored by the UN agency IAEA, but the country has now installed new and more advanced centrifuges there.

According to the IAEA, there were no nuclear substances at the site of the fire and no inspectors were there at the incident.

Probably a cyber attack

Three Iranian government sources, who spoke to Reuters under the promise of anonymity, said last Friday a cyber attack was likely behind Thursday's fire as well.

Iranian authorities have publicly stated that the cause of the fire has been determined, and that it will be made public later.

Israeli Defense Minister Naftali Bennett said on Sunday that Israel is not "necessarily" behind every mysterious incident in Iran.