There are fears of possible further explosions after Wednesday's massive train crash in north-east Iran.
The situation on the ground is still dangerous
Revolutionary Guards have cordoned off the area in case petrol wagons still intact in the wreckage should ignite.
Officials say at least 295 people are known to have died in the blast in the town of Neyshabur but death toll is likely to rise further.
The authorities have announced a day of mourning for Thursday and ordered an investigation into the disaster.
Local residents have been moved away, with officials warning there could be further blasts.
A BBC correspondent in Iran says serious questions are likely to be raised about how such a volatile and deadly mix of materials could have been transported in the same convoy.
According to Iranian TV, the train wagons - which included 17 wagons of sulphur, six wagons of petrol, seven wagons of fertilizers and 10 wagons of cotton wool - broke loose from a train station and rolled about 20km (12 miles).
Then they derailed and caught fire at 0400 local time (0030 GMT) on Wednesday.
Some 200 of the dead are thought to be rescue workers who were tackling the fires when the explosion happened at 0935 (0605GMT).
The blast was so powerful local residents thought it was an earthquake - and Iranian seismologists recorded a quake of magnitude 3.6 at the time of the explosion, AP reported.
Surrounding villages suffered serious damage and officials believe they could find more victims trapped in the ruins of their mud-brick homes.
By nightfall on Wednesday, huge plumes of smoke were still rising from the ashes, and an acrid smell of sulphur hung in the air for kilometres around.
Some sources say the train wagons were set loose by earth tremors.
Iran is still recovering from a devastating earthquake on 26 December in the ancient city of Bam in the south-east of the country, in which more than 40,000 people.
This latest disaster comes at a time of political uncertainty in Iran, with controversial parliamentary elections to be held on Friday.
"The scale of the accident is very extensive and the damage seems to be more than initial estimates suggested," said local official, Vahid Barakche.
"The whole town is shocked by this accident," the editor of a local newspaper, Saeed Kaviani, told AP news agency.
The explosion left an untold number of people injured
"Official vehicles mounted with loudspeakers are roaming the city calling for volunteers to donate blood."
The blast shattered windows more than 10km (six miles) away and could be heard in the regional centre of Mashhad, 75km from the scene.
Top city officials, including the local governor, mayor and fire chief, are believed to be among the dead.
The Iranian news agency (Irna) also said the head of the city's energy department had been killed and that the director-general of the provincial railways was missing.
World's worst train disasters
June 2002: Dodoma region, Tanzania - at least 200 killed when passenger train collides with goods train
Feb 2002: Egypt - 300 killed in fire on train travelling to Cairo
June 1989: Ufa, Russia - More than 400 killed in gas explosion under two trains
Aug 1995: Uttar Pradesh, India - 300 killed in train collision
June 1981: Bihar, India - 800 killed when cyclone blows train into river
An Irna reporter was also killed in the blast.
Several hundred injured people were taken to hospitals in Neyshabur and Mashhad.
An official from the Natural Disasters Headquarters in Neyshabur said the hospitals had been overwhelmed with casualties.