By Abinaya V and Jatindra Dash

June 3 (Reuters) - 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.

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

"I was there at the site and I can see blood, broken limbs and people dying around me," an eyewitness told Reuters by phone.

The collision occurred at about 19:00 local time (1330 GMT) on Friday when the Howrah Superfast Express, running from Bangalore to Howrah, West Bengal, collided with the Coromandel Express, which runs from Kolkata to Chennai.

Authorities have provided conflicting accounts on which train derailed first to become entangled with the other and have yet to make any statements about possible causes.

An extensive search-and-rescue operation has been mounted, involving hundreds of fire department personnel, police officers and 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.

(Reporting by Abinaya Vijayaraghavan and Akriti Sharma in Bengaluru, by Jatindra Dash in Bhubaneswar and by Subrata Nag Choudhury; Editing by Edwina Gibbs)