Several carriages were completely destroyed in the collision on a single-track line near Andria in the southern region of Puglia, Italy. Photo: AP

At least 20 people have died and dozens are injured after two passenger trains collided in the middle of an olive grove in Italy's south-east.

Police said the accident happened on a single stretch of track between the towns of Corato and Andria at 11.30am local time.

One child, believed to be aged 7, was pulled from the wreckage and taken to hospital in a helicopter, as more than 100 ambulances waited for more victims to be rescued, Italian media reported.

Carriages were left crumpled after two commuter trains collided head-on in the southern region of Puglia, killing several people. Photo: Italian Firefighter Press Office

An aerial image showed train carriages were smashed and crumpled by the head-on collision, with debris spread out on either side of the track.

In the hours after the crash, officials updated the death toll from 11 to 20 and said dozens of people had been taken to hospital, some with serious injuries.

A local blood bank, Avis Corato, posted on social media to urge people with O Positive blood to donate at Andria Hospital following the train tragedy.

Firefighters inspect the wreckage, where it's believe some people remain trapped. Photo: Italian Firefighter Press Office

"The blood transfusion centre of Andria will perform an extraordinary collection of blood this afternoon," the blood bank's post said.

Firefighters and paramedics arrived quickly after the two four-carriage trains collided, with footage taken on mobile phones showing ambulances pulling up in a leafy olive grove as people rushed towards the trains with spinal boards.

Rescuers climbed among the bent metal and smashed glass of the two trains to get to trapped passengers, which reportedly include university students and children.

Wreckage of carriages at the scene of a train accident where 20 people died. Photo: Italian Firefighter Press Office

The trains collided with such force that three carriages were reduced to rubble, with one derailed, and nearby olive trees ripped from the ground leaving their roots exposed.

Matteo Renzi, the country's prime minister, was in Milan at the time of the crash but said he would return to Rome immediately to address the crisis.

"[This] is a moment of tears," he said. "We won't stop until we get a clear explanation over what happened."

News agency AFP quoted investigators as saying the crash was possibly caused by human error.

Local fire brigade commander Riccardo ZingaroI told Italy's public broadcaster Rai TV that "there are many dead".

"There was a head on collision on a single track, several carriages were completely crushed and the emergency workers are pulling people out of the train, there are many injured," he said.

Massimo Mazzilli, who works in local government in the town of Corato, said on Facebook the scene looked like a plane had crashed. "It's a disaster," he wrote. "Unfortunately there are victims."

A photographer at the scene, Gaetano Loporto, told the BBC that the wreck had been cordoned off by police as they tried to help those who remained on the train.

"Two trains crashed in front of me, on a curve, so there's no possibility to see each other," he said.

Several hours after the crash, passengers were still being pulled from the wreckage alive, the BBC reported.

Graziano Delrio, Italy's Minister for Infrastructure, and Fabrizio Curcio, the head of Civil Defence, headed to the crash site "to ensure coordination of relief efforts", a statement by the government's Presidency of the Council of Ministers said.

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

with Reuters, AP