Iranian news agencies on Saturday reported a large explosion in the sky above the town of Natanz, which houses nuclear sites, but the incident was later downplayed by state TV.
The country's air defence units fired a missile as part of an exercise, said state media, after local residents reported hearing a large blast.
The missile was fired to test a rapid-reaction force over Natanz, reports said. “Such exercises are carried out in a completely secure environment … and there is no cause for concern,” Army spokesman Shahin Taqikhani told state TV.
The exercise came as talks to avoid the collapse of the 2015 Iran nuclear accord in Vienna drew to a close after five days.
US Secretary of State Antony Blinken said on Friday that Iran “does not seem to be serious” about returning to the pact, while European countries on Saturday voiced their “disappointment and concern”. The seventh round of talks is due to resume next week. An unnamed senior Iranian official said on Sunday that Tehran did not believe the US would lift sanctions.
Iranian news agencies had earlier reported the explosion, saying there was no official explanation of the incident, which caused a flurry of speculation on social media that an attack on Iranian nuclear sites could be under way.
The semi-official Fars news agency quoted its reporter in nearby Badroud as saying a short blast was heard which was accompanied by an intense light in the sky.
“No exact details are available about this,” Fars quoted the local governor of Badroud as saying.
Attacks on Natanz
In April, Iran accused Israel of sabotaging the Natanz nuclear site and vowed revenge for an attack that appeared to be the latest episode in a long-running covert war.
Israeli media outlets have quoted unnamed intelligence sources as saying the country's Mossad spy service carried out a successful sabotage operation at the underground Natanz complex, potentially setting back enrichment work there by months. But Israel has not formally commented on the incident.
Israel fears that Iran may soon have enough uranium enriched to “weapons grade” to build a nuclear device, and the country's air force has rehearsed a pre-emptive attack on Iranian nuclear facilities, in the event that sabotage operations fail. Natanz houses thousands of advanced centrifuges for uranium enrichment.
Addressing the UN General Assembly in September, Prime Minister Naftali Bennett said: “Iran’s nuclear program has hit a watershed moment, and so has our tolerance. Words do not stop centrifuges from spinning … We will not allow Iran to acquire a nuclear weapon.”
Aware of the risk of a pre-emptive Israeli strike, Iran has invested heavily in air defences. In 2017, Iranian Brig Gen Farzad Esmaili said that a top priority was protecting nuclear facilities. That year, Iran conducted its first tests of the Russian-made S-300 air defence system, which the Iranian military purchased for an estimated $800 million.
Iran also has a large number of older surface to air missiles including the Russian-made SA-3, which can be fired at targets up to 80,000 ft. The SA-3 successfully shot down a $200 million US Global Hawk drone in June 2019 — a high altitude but slow flying aircraft. Another system, the SA-15, was linked to the shooting down of Ukraine International Airlines Flight 752 on January 8, 2020, with the loss of 176 lives.
In the event of a conflict, it is unclear how Iran's air defences might fare against Israeli F-35s. Like Iran, Syria has deployed S-300 and SA-3 missiles to stop Israeli incursions into its air space, but has had little success stopping air raids targeting Iran-backed groups and weapons.
For this reason, many of Iran's nuclear facilities are located deep underground.