Twenty-six people were killed and at least 85 injured after two trains collided head-on in Greece late on Tuesday, the fire brigade said, while the circumstances of the crash remained unclear.

A passenger train travelling from Athens to the northern city of Thessaloniki collided with a cargo train outside the city of Larissa in central Greece, said the governor of the Thessaly region.

"The collision was very strong," governor Konstantinos Agorastos told SKAI TV, adding the first four carriages of the passenger train had derailed.

The first two carriages, which caught fire after the collision, were "almost completely destroyed," said Agorastos.

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.

"There was panic in the carriage, people were screaming," a young man who was evacuated to a nearby bridge told SKAI TV.

"It was like an earthquake," Angelos Tsiamouras, another passenger, told ERT.

Broadcaster SKAI showed footage of derailed carriages, badly damaged with broken windows and thick plumes of smoke, as well as debris strewn across the road. Rescue workers were seen carrying torches in carriages looking for trapped passengers.

"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.