At least 22 people died and 87 were injured when a passenger train travelling from the Iranian city of Mashhad to Yazd derailed on Wednesday, the semi-official Fars news agency reported.
The report said the number of casualties could rise, although initial details about the disaster remained unclear.
The train was reportedly carrying about 350 passengers.
More than 50 Red Crescent medics, 10 ambulances and three relief helicopters were sent to the accident site.
Iran’s President Ebrahim Raisi offered condolences over the crash and announced an investigation would be undertaken into its causes. On Wednesday night, authorities ordered the arrest of six people allegedly involved in causing the crash, though they released no other information about why they were suspected.
The derailment happened about 50 kilometres away from Tabas at 5.30am, on the rail that links the town to the central city of Yazd, state-controlled Irib News reported.
Tabas lies about 550km south-east of the capital, Tehran.
“Two teams of Red Crescent aid workers have been sent to the scene of the accident, which is located within 50km of the Tabas axis,” Irib said.
The train crashed into an excavator near Mazino station, according to reports.
Iran’s President Ebrahim Raisi offered condolences over the crash and announced an investigation would be undertaken into its causes. On Wednesday night, authorities ordered the arrest of six people allegedly involved in causing the crash, though they released no other information about why they were suspected.
Initial reports showed that three of seven wagons were derailed, reported Irib, citing the head of the Red Crescent Society for Rescue and Relief.
State TV later aired images from a hospital where the injured received treatment. One of the injured told the broadcaster they felt the train suddenly brake and then slow down before the derailment occurred.
“Passengers were bouncing in the car like balls in the air,” said the passenger.
Rescue efforts involved three helicopters from Birjand, Mashhad and Yazd, as well more than 10 ambulances, said Mojtaba Khaledi, the country's emergency services spokesman.
The crisis management chief of South Khorasan province said five ambulances were already at the accident scene, with 12 more on the way.
Mohammad Eghbali, the public relations manager of the Yazd Red Crescent Society, said a helicopter with four teams from Iran’s Rapid Reaction Brigades was dispatched from Yazd and Ardakan counties.
Mr Khaledi said 30 people had been admitted to hospital.
He said the officers at the scene of the accident were covering dead bodies “because the severity of the accident is so high, some of the bodies cannot be identified”.
“Three forensic vehicles are being sent to the scene to transfer the bodies for forensic testing as soon as possible to identify the bodies,” Mr Khaledi told Mashregh News agency.
Aerial footage of the accident site showed the front two cabins lying on their side while the next three were damaged, according to a video released by the Young Journalists Club news agency.
Iran’s worst train disaster came in 2004, when a runaway train loaded with petrol, fertiliser, sulphur and cotton crashed near the historic city of Neyshabur, killing about 320 people, injuring 460 others and damaging five villages.
Another train crash in 2016 killed dozens of people, leaving scores injured.
