A massive explosion at a military arms depot near the Iranian capital Tehran killed 27 elite Revolutionary Guards on Saturday and wounded 16 others, state broadcaster IRIB said.
Officials said it was an accident which happened as troops were moving munitions at a base in Bidganeh, near the town of Shahriar, some 45 km (28 miles) west of Tehran.
While there was no indication of any attack, the explosion shook homes and rattled windows for miles around at a time of mounting tensions with Israel over Iran's nuclear programme.
So far, 27 personnel of Revolutionary Guards have been martyred and 16 others wounded, some of them are in a critical condition, IRIB quoted Revolutionary Guards spokesman Ramezan Sharif as saying.
Today at 13:30, (0900 GMT), an explosion happened in one of the Revolutionary Guards' bases while a consignment of explosive devices was being moved out from the arsenal, besides that some munitions in the arsenal exploded which created a terrifying sound, Sharif said.
Residents in western suburbs of Tehran told Reuters they had felt the blast, with some assuming it to be a low-level earthquake.
Fire at the base 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)