Italy's government is sending 900 soldiers and police to mafia strongholds in the south after six African immigrants were murdered there last week in what the authorities believe was a show of force by the crime syndicates.

Interior Minister Roberto Maroni said on Tuesday the cabinet had approved deploying 500 soldiers to the areas where there is organised crime activity, in addition to the 400 extra police sent to the Campania region where the killings took place.

The six victims -- from Ghana, Togo and Liberia -- were shot dead on Thursday at a tailor's shop in Castelvolturno, a town near Naples, in one of the bloodiest shootouts in recent memory by the Camorra, the local version of the mafia.

Investigators believe the hitmen were a group of young, mafiosi from the Casalesi clan asserting control over drug and extortion rackets.

Witnesses said the killers, some of whom wore police vests, sprayed 130 bullets at their victims then fired in the air to celebrate.

One of the suspected hitmen, 29-year-old Alfonso Cesarano, was arrested on Thursday. He had previously been under house arrest for drug trafficking.

Investigators initially said the attack was part of a drug-related turf war, but they did not rule out the immigrants may have been killed randomly to intimidate residents.

Fellow immigrants in Castelvolturno rioted on Friday calling for justice and saying the victims had nothing to do with drugs.

"The Casalesi want to scare people so they stop cooperating with the judiciary, force businessmen to pay protection money and make immigrants move elsewhere because they are bothering them," said Franco Roberti, Naples' top anti-mafia prosecutor.