At least 17 people have been killed and more than 50 injured after a passenger train part-derailed and crashed on the Mashhad to Yazd line in the early hours of Wednesday. 

Four of the train's seven cars, carrying an estimated 350 people, came off the tracks in the dark at about 5.30am near the desert city of Tabas, South Khorasan province. 

Islamic Republic of Iran Railways and ISNA news agency both initially reported the train had struck an excavator on the tracks. It was later understood that the train's undercarriage had struck the rails instead, though the exact cause is still being investigated.

Emergency services spokesman Mojtaba Khaledi confirmed the death toll stood at 17 by late morning, adding that over a dozen of the wounded had suffered "critical injuries" and been transferred to local hospitals.

Aerial footage of the site of the disaster showed the cars on their side, with apparently volunteer rescuers running toward the scene in a bid to care for the injured.

"The train was moving when it suddenly braked hard," a passenger told ISNA. "People lost their balance and bounced off the surroundings like balls. The car overturned; the window of the one we were in broke and we were able to get out."

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