Eighty-five people have been injured.

The cause of the deadliest rail crash Greece has seen in decades remains unclear.

An intercity passenger train travelling from Athens to the northern city of Thessaloniki and a cargo train collided at high speed outside the city of Larissa in central Greece.

The impact caused a fire in a number of the passenger carriages, burning many commuters who were rushed to hospitals.

"We heard a big bang, [it was] 10 nightmarish seconds," Stergios Minenis, a 28-year-old passenger who jumped to safety from the wreckage, said.

"We were turning over in the wagon until we fell on our sides ... then there was panic, cables [everywhere], fire, the fire was immediate, as we were turning over we were being burned, fire was right and left."

Thessaly regional governor Konstantinos Agorastos told SKAI TV that the first four carriages of the passenger train were derailed in the crash, while the first two carriages, which caught fire, were "almost completely destroyed".

He said the two trains hurtled towards each other on the same track.

"They were travelling at great speed and one [driver] didn't know the other was coming," the governor said.

About 250 passengers were evacuated safely to Thessaloniki on buses. One passenger told state broadcaster ERT he managed to escape after breaking the train window with his suitcase.

Broadcaster SKAI showed footage of derailed carriages, badly damaged with broken windows and thick plumes of smoke.

Footage also showed debris strewn across the road with rescue workers carrying torches looking for trapped passengers inside the carriages.

"The evacuation of passengers is under way in very difficult conditions given the severity of the collision of the two trains," fire brigade spokesperson Vassilis Varthakogiannis said in a televised address.

Early on Wednesday, footage from state broadcaster ERT showed rescue workers with headlights searching the wreckage and surrounding fields for survivors.

Local media reported about 350 people were travelling on the passenger train. The cargo train had been travelling from Thessaloniki to Larissa.

Greece's ageing railway system is in need of modernising, with many trains travelling on single tracks while signalling and automatic control systems are yet to be installed in many areas.

AAP