Pope Francis urged the killers of a three-year-old boy murdered in southern Italy as part of an apparent mafia hit to repent after a crime that he said was unprecedented.

The charred body of Nicola Campolongo, known as "Coco", was found along with that of his grandfather Giuseppe and a young Moroccan woman in a burnt-out car in the town of Cassano Jonio near Cosenza in Calabria last week.

According to Italian media reports, investigators believe the boy's 52-year-old grandfather Giuseppe Iannicelli was the real target of mafia killers who shot all three people in the car in the head before apparently torching the vehicle.

"This violence against such a small child seems without precedent in the history of crime," the pope said during his weekly Angelus address in St Peter's Square.

"Let us pray with Coco, who is certainly with Jesus in heaven and for the people who carried out this crime, so that they repent and convert to the Lord," he said.

The murder of the 3 year-old was a shock even in a country long accustomed to mafia violence.

Italian media reported that Iannicelli was linked to drug trafficking, a speciality of 'Ndrangheta, the Calabrian mafia that has seized control of much of the trade and which is seen as one of the world's most dangerous criminal organisations.