At least 21 people died and a further 87 were injured when a passenger train travelling through eastern Iran struck an excavator and derailed nearly half its cars, officials have said.

The train derailment near the desert city of Tabas was the latest disaster to strike the Islamic Republic in recent weeks as Tehran struggles under US sanctions and any return to its nuclear deal with world powers remains in doubt.

The train operated by the state-run Islamic Republic Railway was carrying some 350 people as it travelled from Tabas, 340 miles (550km) southeast of Tehran, to the city of Yazd. The route had begun as an overnight train out of Iran’s holy city of Mashhad.

Based on images after the crash, it appeared the train’s locomotive passed the excavator and the later cars somehow hit the digger and caused the derailment, although authorities did not immediately explain how the disaster happened in the rural scrubland near a railway bridge.

“Passengers were bouncing in the car like balls in the air,” one unnamed injured passenger told Iranian state television.

The state-run IRNA news agency gave the casualty figures, citing emergency officials.

Rescue teams with ambulances and helicopters arrived in the remote area where communication is poor. Over a dozen people suffered critical injuries, with some transferred to local hospitals, officials said.

Aerial footage of the desert site of the disaster showed train cars on their side, with some rescuers running at the scene as they tried to care for the injured.

State TV later aired images from a hospital where the injured were receiving treatment. One of those injured told the broadcaster they felt the train suddenly brake and then slow before the derailment.

The incident happened some 30 miles (50km) outside of Tabas.

The report said the crash was under investigation.

Initial reports suggested the train collided with an excavator near the track, although it was not immediately clear why an excavator would have been close to the railway line at night. One official suggested it could have been part of a repair project.

Iran’s president Ebrahim Raisi offered his condolences over the crash and said an investigation would be held into its causes.

The country’s worst train disaster happened in 2004, when a runaway train loaded with petrol, fertiliser, sulphur and cotton crashed near the historic city of Neyshabur, killing some 320 people, injuring 460 others and damaging five villages. In 2016, a train collision in northern Iran killed at least 43 people and injured about 100.

Iran has some 8,700 miles (14,000km) of railway lines sending both people and goods across the country, particularly in rural areas.

The country also has some 17,000 annual deaths on its roads, one of the world’s worst traffic safety records. The high toll is blamed on wide disregard for traffic laws, unsafe vehicles and inadequate emergency services.

Iran, already straining under US sanctions over its collapsed nuclear deal, has been mourning the deaths of at least 41 people killed in a building collapse in the country’s southwest.