Iranian authorities are investigating after an explosion east of the capital near a site linked to the regime's nuclear testing programme.
A bright and large flash of light was seen in the night sky over Tehran early on Friday in images shared widely on social media, Iran's Fars news agency reported.
"In the early hours after midnight on Friday, a number of social media users reported seeing an orange light in the eastern part of Tehran," said Fars.
"In the videos sent by (our) readers, this light is seen for a few seconds," it reported, adding it was following up the issue with the relevant authorities.
Fars said later that the flash was caused by "an industrial gas tank explosion" near a facility belonging to the defence ministry.
— Farnaz Fassihi (@farnazfassihi) June 25, 2020
The blast appears to have come from near Parchin, a military site.
Davoud Abdi, a defense ministry spokesman, told state TV that the blaze was quickly taken under control and that there were no casualties.
Fars cited an "informed source" and insisted the site of the incident was not related to the military.
"At the entrance of the (nearby) Parchin military zone, there is no movement or traffic of fire and rescue vehicles," Fars added.
Mehr news agency had said a "terrible sound" was heard.
BREAKING: There are videos circulating of what appears to be an explosion in Pardis, south Tehran, tonight. This video, I think, was shot in Bame Tehran, north of the city. You can clearly see what appears to be a fireball that turns the sky red
— Shayan Sardarizadeh (@Shayan86) June 25, 2020
"The cause of this sound and light is not yet known, but it was clearly heard in Pardis, in Boumhen and surrounding areas" of the Iranian capital.
State television said various institutions were investigating the issue. Fars said police were involved in the investigation.