Iranian air defenses 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 defense 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.