At least 21 people were killed and dozens injured Wednesday when a train derailed near the central Iranian city of Tabas after hitting an excavator beside the track, state media reported.

The train was on its way from the northeastern city of Mashhad to the central city of Yazd and was carrying 348 passengers, according to the national rail authority, when it careered off the track in the desert at 5:30 am (0100 GMT).

Yazd province's emergency organisation chief Ahmad Dehghan said 21 people were killed in the incident, according to the website of state broadcaster IRIB, revising an initial death toll of 17 given by national authorities.

Earlier, national emergency services spokesman Mojtaba Khaledi told state television that 86 people were injured.

Khaledi had noted that the number of dead may rise, as some injured were in "critical condition". Twenty-four ambulances and three helicopters -- seen on state television airlifting casualties to hospital -- were dispatched to the scene, he said.

Tabas is located in South Khorasan province, roughly 900 kilometres (560 miles) by road from Tehran. The provincial judiciary said arrest warrants have been issued for six people in connection with the accident.

The train "derailed after hitting an excavator" that was near the track, Mir Hassan Moussavi, the deputy head of Iran's state-owned railways,  told the state broadcaster.

Rescue teams inspected the overturned carriages as onlookers gathered nearby, pictures posted by the ISNA news agency showed.

One of the pictures showed a yellow excavator on its side by the track.

Five of the train's 11 coaches came off the rails, the Iranian Red Crescent's head of emergency operations, Mehdi Valipour, told state television.

Investigation 

President Ebrahim Raisi expressed his grief over the derailment and offered his condolences to the families of the dead.

He also issued orders for a speedy investigation into the causes of the crash, his office said.

Roads and Urban Development Minister Rostam Ghassemi apologised to Iranians on Twitter and said the ministry was responsible for the incident.

"Hitting the excavator was the main cause of the accident," Ghassemi said, adding that initial reports indicated that the driver only braked after hitting the obstructing vehicle. 

"What is clear is the human error, which is more on the part of the driver, and why and how should be investigated more closely," he stressed.

The Tabas prosecutor visited the scene as a judicial investigation was launched, Iranian media reported.

The train derailment comes after a tower block collapsed in southwestern Iran last month killing at least 43 people.

The collapse of the 10-storey Metropol building, which was under construction in Abadan, sparked angry protests in solidarity with the families of the dead.

The provincial judiciary said it had arrested 13 people, including Abadan's mayor and two former mayors, suspected of being "responsible" for that tragedy.

The disaster was one of Iran's deadliest in years and sparked demonstrations across the country against authorities accused of corruption and incompetence.

In 2016, two trains collided and caught fire in northern Iran, killing 44 people and injuring dozens.

The then head of Iranian railways resigned after four of his employees were arrested following the collision on the main line between Tehran and second city Mashhad.