Iranian President Mohammad Khatami Thursday ordered a tough investigation into how railway wagons loaded with highly explosive cargoes managed to run away and explode, as the death toll rose to 320.
Describing Wednesday's freak disaster as "sad and unacceptable", the embattled president ordered "full light to be shed on the causes of the accident", the official news agency IRNA said.
"I hope that with more attention and responsibility we will no longer witness such sad and unacceptable accidents," said Khatami, as recovery teams and mechanical diggers began clearing away the blackened wreckage of the wagons.
The provincial head of the Iranian Red Crescent, Seyed Ali Hosseini, gave the new toll of 320 dead and 460 injured, adding, "It is possible that there are more bodies under the wreckage."
He said that recovery teams still had to search through a crater 150 metres (500 feet) in diameter and 20 metres deep.
Vahid Barakchi, the head of Khorassan province's disasters unit, earlier told AFP, "We currently have a death toll of 295. If we find more bodies, I don't think it will be more than 10 or 15."
The blast occurred after a string of 51 wagons ran away in the early hours of Wednesday and derailed at Khayyam station near Neyshabour, some 75 kilometres (50 miles) from the north-eastern city of Mashhad.
The cargo of sulphur, fertiliser, petrol and cotton blew up as fire-fighters - surrounded by crowds of curious villagers also apparently unaware of the deadly cargo - were attempting to douse initial fires.
The deadly mix of cargoes is likely to raise tough questions over rail transport safety. IRNA reported that nearby railway station managers had been called in for questioning.
Although the alert level around the wreckage has been lowered, the station remained sealed off by members of the elite Revolutionary Guards. Recovery personnel wore face masks at the scene to avoid inhaling potentially toxic fumes.
The government declared Thursday a day of mourning in Khorassan province, as grieving relatives searched for their loved ones by scouring lines of charred corpses.
At the Behesht Fazl cemetery in Neyshabour, an AFP correspondent counted about 190 corpses and body parts lined up on the ground and covered in plastic.