A huge explosion has rocked a military base west of Iran's capital, Tehran, officials say.
Several people were killed after the blast inside a Revolutionary Guards weapons depot, according to the semi-official Fars news agency, but this has not been confirmed.
Windows in nearby buildings have been shattered and the blast was heard in central Tehran, 40 km (25 miles) away.
A helicopter and ambulances have reportedly been sent to the scene.
Local MP Hossein Garousi said "a large part of an ammunition depot exploded," parliament's website reported.
It is not clear what caused the explosion in the village of Bidganeh, near the city of Karaj.
Karaj resident Kaveer told the BBC's Newshour programme that the sound was "deafening".
"We were kind of shocked. I just ran out of the house and looked around," he said.
An elite military force, the Revolutionary Guard was set up shortly after the 1979 Iranian revolution to defend the country's Islamic system.
It has since become a major military, political and economic force in Iran.
The Revolutionary Guard has been targeted by UN sanctions aimed at pressuring Iran to halt uranium enrichment.
There have been occasional unexplained explosions in Iran before.
Eighteen people were killed in a blast at a Revolutionary Guards base in the north-western Lorestan province in October 2010.
The latest blast comes at a time of heightened tensions over Iran's nuclear ambitions.
The UN's nuclear watchdog the IAEA released a report on Tuesday which, correspondents say, prompted new fears that Iran's nuclear programme has a military objective.
There has also been speculation in Israel's media that Prime Minister Benjamin Netanyahu is considering ordering strikes against Iran's nuclear sites, in the hope of stalling or ending its programme.
Iran says its nuclear programme has purely civilian aims.