An Iranian defence industry body said two workers were killed in a fire at an explosives factory in an eastern district of Tehran, the official IRNA news agency reported on Monday.
An Iranian opposition website, Saham, described the incident as a strong explosion and said it took place near Iran's sprawling Parchin military facility, which is located around 30 km southeast of the capital. It did not give a source for its report, which could not be independently verified.
Iran's Defence Industries Organisation said the fire broke out on Sunday evening, IRNA said, giving no further detail.
The UN nuclear watchdog agency suspects Iran around a decade ago may have carried out explosives tests at Parchin that could be relevant for any attempt to develop a nuclear weapons capability. The IAEA wants to visit a specific location at the site, but Iran has so far not granted access.
Iran says Parchin is a conventional military facility and that its nuclear programme is entirely peaceful. It has often accused its enemies of seeking to sabotage its atomic activities.
Israel and the United States have not ruled out military action against Iran if diplomacy fails to resolve a decade-old dispute over Tehran's nuclear programme. Israel is widely believed to be the Middle East's only nuclear-armed power.
Three years ago, Iran said a massive explosion at a military base 45 km (28 miles) west of Tehran killed 17 Revolutionary Guards, including the head of the elite force's missile programme. It said the blast was caused by an accident while weapons were being moved.