Pope Francis has urged Italian mobsters to pray for forgiveness after a child was shot in the head and killed in what authorities describe as an 'unprecedented' Mafia hit.

In his weekly Angelus address at St Peter's square, the Holy Father urged the killers to "repent" for the heinous crime that has shocked Italy, a country accustomed to mafia crime.

"This violence against such a small child seems without precedent in the history of crime," Pope Francis said. "Let us pray for 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."

Download the new Independent Premium app Sharing the full story, not just the headlines

The body of three-year old Nicola Campolongo was found on 17 January along with that of his grandfather Giuseppe Iannicelli , 57, and a young Moroccan woman, Ibtissa Touss, 27, in a torched Fiat Punto in the town of Cassano Jonio in Calabria, southern Italy.

"In all my years investigating organized crime murders, none has been as horrific as this one," lead prosecutor Franco Giacomantonio told reporters. "It is unimaginable that a child can be made to pay for the crimes of his parents."

Italian authorities believe the 'Ndrangheta, Italy's most feared crime group, is behind the triple homicide. Police found a 50 euro cent coin on the roof of the burnt-out car, a symbol of vendetta, which suggests the hit was carried out in retaliation for an unpaid drug debt.

According to Italian media, Iannicelli was the real target of mafia killers who shot all three people in the car in the head. The 57-year been convicted of dealing narcotics and was serving an eight-year sentence under nocturnal house arrest. Authorities were alerted when he failed to check in.

Coco's 24-year-old mother, Antonia Iannicelli, is serving a four-year jail sentence for possession and sale of drugs. She did not attend the funeral fearing her life would be in danger. Her two other children, ages four and five, are in protective custody.

No suspects have been named in the triple homicide. The investigation continues.