At least 20 people died and dozens were injured when two passenger trains collided head-on in southern Italy on Tuesday.

The crash happened on a single stretch of track in countryside between the towns of Corato and Andria. Both trains were made up of four carriages.

"We are working with dozens of rescue teams to open up the carriages," said Luca Cari, a fire service spokesman, adding they had saved a young boy from the wreckage.

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"The rescue is complicated because this happened in the middle of the countryside," he said.

There was no immediate reason given for the collision, which took place at around 11.30 am on a hot summer's day in the region of Puglia.

"We won't stop until we get a clear explanation over what happened," Prime Minister Matteo Renzi told reporters, cutting short a visit to Milan in northern Italy to return to Rome.

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A photograph handed to reporters by the fire brigade showed just four carriages left standing upright on the rails. A fifth carriage had jumped off the tracks, while the other three units appeared pulverized.

"It looks like there has been a plane crash," said the mayor of Corato, Massimo Mazzilli.

The stretch of track is operated by a regional rail company Ferrotramviaria. It was not clear how many people had been on the trains.

The last major rail disaster in Italy was in 2009 when a freight train derailed in Viareggio, in the center of the country, with more than 30 people living close to the tracks killed in the subsequent fire.