DUBAI, Dec 4 (Reuters) - Iranian air defences fired a missile as part of an exercise on Saturday over the central town of Natanz, which houses nuclear installations, state TV reported, after local residents reported hearing a large blast. The TV said air defence units fired the missile to test a rapid reaction force over Natanz. "Such exercises are carried out in a completely secure environment ... and there is no cause for concern," Army spokesman Shahin Taqikhani told the TV. Iranian news agencies earlier reported a large explosion in the sky above Natanz, but said there was no official explanation of the incident. 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. The Islamic Republic says its nuclear ambitions are peaceful. In April, Iran accused arch-foe Israel of sabotaging its key 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 opposed the original 2015 Iran nuclear accord with world powers, which aimed to curb Iran"s nuclear programme in return for lifting sanctions against Tehran, as too limited in scope and duration. Israeli leaders have long threatened military action against Iran if they deem diplomacy a dead end for denying it nuclear weaponry. read more