A massive explosion at a military arms depot near the Iranian capital Tehran killed 15 servicemen on Saturday and wounded at least 25 others, in what officials were treating as an accident.
While there was no indication of any attack, the explosion shook Iranians for miles around at a time of mounting tensions with Israel over Iran's nuclear programme.
A spokesman for the Revolutionary Guards -- Iran's elite military force -- said the blast happened as troops were moving munitions at a base in Bidganeh, near the town of Shahriar.
My dear colleagues in the Revolutionary Guards were moving munitions in one of the arsenals at that base when, due to an incident, an explosion happened, Ramezan Sharif told state TV news channel IRINN.
Some of the wounded are reported to be in a critical condition, he added. The semi-official Fars news agency said 25 people had been taken to hospital.
Residents in western suburbs of Tehran, 45 km away, told Reuters they had felt the blast, with some assuming it to be a low-level earthquake.
The explosion happened around noon (0830 GMT) and the fire continued to rage for hours. Surrounding streets were closed and reporters were kept away from the scene.
Some media reported there had been two explosions and the head of Iran's Red Crescent organisation said there was a risk of further blasts.
Mahmoud Mozafar told the Mehr news agency that only six paramedics had been allowed into the Amir Al-Momenin military base and that thick smoke was hampering the rescue operation.
There were no reports linking the blast to any air strike or other attack. Tension has risen in recent weeks between Iran and its enemies Israel and the United States, which have not ruled out striking facilities they believe are working towards making nuclear weapons.
Sharif denied what he said was speculation in the Western media that the military base was linked to Iran's nuclear programme.
This blast is not related to any nuclear tests that some foreign media have reported, he told Mehr.
Tehran denies Western accusations, that were given some credence by a report from the UN nuclear agency this week, that its nuclear programme has military ends.
On October 12 last year a similar blast at a Revolutionary Guards munitions store killed and wounded several servicemen in Khoramabad, western Iran. Authorities said that explosion was an accident.
(Additional reporting by Mitra Amiri; Writing by Robin Pomeroy; Editing by Jon Hemming)