Carriages completely destroyed in a head-on collision involving two passenger trains near Andria on July 12, 2016 in Puglia, Italy. 25 people have been killed and dozens more hurt in the collision.The emergency services have been trying to free passengers from the shattered carriages. [Photo/VCG]

BARI, Italy - At least 25 people died and dozens were injured on Tuesday when two passenger trains collided head-on at high speed in southern Italy, sending debris flying into surrounding olive groves.

Three carriages were torn apart by the violence of the impact after the two trains hit each other while travelling down the same stretch of track linking the small towns of Corato and Andria in the southeastern Puglia region.

Rescue operators raised the death toll to 25 on Tuesday evening after a day of frantic rescue operations, and warned that it could climb higher as some of the 50 injured were in serious condition.

Authorities urged blood donors to go to local hospitals.

There was no immediate indication of what had caused one of Italy's worst train disasters in recent years, but the government promised a full and swift investigation.

"Tears and grief for the victims and their families, but also a lot of anger. We demand clarity over what happened in Puglia this morning," Prime Minister Matteo Renzi said on Twitter, before making his way to the region to meet with rescuers and local politicians.

"We will remain at the side of the people of Puglia in this moment of great pain and desperation," he said in the evening, adding that Transport Minister Graziano Delrio would report to parliament on the accident on Wednesday.