At least 233 people were killed and 900 were injured when two passenger trains collided in India's Odisha state, a government official said on Saturday, making the rail accident the country's deadliest in more than a decade.

The death toll from Friday's crash is expected to rise, the state's Chief Secretary Pradeep Jena said on Twitter.

He added that over 200 ambulances had been called to the scene of the accident in Odisha's Balasore district and 100 additional doctors, on top of 80 already there, had been mobilised.

Early on Saturday morning, Reuters video footage showed police officials moving bodies covered in white cloths off the railway tracks.

Video footage from Friday showed rescuers climbing up one of the mangled trains to find survivors, while passengers called for help and sobbed next to the wreckage.

The collision occurred at about 7pm local time (9.30pm Malaysian time) when the Howrah Superfast Express, running from Bangalore to Howrah, West Bengal, derailed and became entangled with the Coromandel Express, which runs from Kolkata to Chennai, railway officials said.

Authorities have provided conflicting accounts on which train derailed first to become entangled with the other. The Ministry of Railways said it has initiated an investigation into the incident.

Although Jena and some media reports have suggested a freight train was also involved in the crash, railway authorities have yet to comment on that possibility.

An extensive search-and-rescue operation has been mounted, involving hundreds of fire department personnel and police officers as well as sniffer dogs. National Disaster Response Force teams were also at the site.

On Friday, hundreds of young people lined up outside a government hospital in Odisha's Soro to donate blood.

According to Indian Railways, its network facilitates the transportation of over 13 million people every day. But the state-run monopoly has had a patchy safety record because of ageing infrastructure.

Odisha's Chief Minister Naveen Patnaik declared a day of state mourning on June 3 as a mark of respect to the victims.

- Reuters