Italy sent fresh riot police reinforcements on Friday to secure a council estate in its southern virus "red zone", where nearly 50 cases of new coronavirus cases among foreign farm workers has sparked tensions with locals on Thursday.

Some 700 people were ordered to remain indoors this week in the complex of five blocks of flats in Mondragone, 60 km north of Naples, for 15 days, while local health authorities test them for the virus, the region's head, Mr Vincenzo De Luca, said late on Thursday.

The local health authorities said 43 positive cases had been identified and tests were being carried out on all the residents.

Four of the high-rise blocks house undocumented Bulgarian workers who feared losing their jobs, while Italian squatters occupy the fifth, Mr De Luca said.

The estate is "one of the thousands of ghettos in Italy, where we amass more or less undocumented foreigners to make them live in more or less heinous conditions", said Corriere della Sera's editorial writer Goffredo Buccini.

Fifty soldiers were sent in to help secure the zone on Thursday after clashes between frustrated Bulgarians who wanted to return to work to earn money for food and angry locals who blamed them for spreading the virus.

Hundreds of Bulgarians who came out to demonstrate on Thursday were persuaded by police to return inside, but locals who learnt that they had left the Palazzi Cirio estate then turned up to hurl stones and trash cars, local media reported.

Coronavirus tests were being offered to residents living near the estate and if 100 cases surfaced, the whole seaside town would be locked down, Mr Vincenzo De Luca said.

He said a few people with the virus had since slipped through the net and disappeared, but insisted surveillance of the estate would be 24 hours non-stop from now on.

"The Bulgarian worker s... are part of the endless labor force working in the southern countryside without rights, often without contracts, without any security," organized crime expert Roberto Saviano wrote in the Repubblica daily on Friday.

No one was supposed to say that those spreading the disease are the foreigners, the invaders, the immigrants, the families of Bulgarian workers accused of going out to continue working, he said.

"But it would have happened the same if it had been Italian workers living in those working conditions, with those wages".

The estate was built decades ago as part of a project to transform Mondragone into the world mozzarella capital, he said.

Furious locals "seem to forget that those workers are essential to the countryside, that without them the (mozzarella-producing) buffaloes would drown in shit and neglect", he added.

It was not the only cluster of new cases in Italy, which lifted its lockdown at the start of June after three months of a pandemic which has officially killed over 34,600 people.

At least 64 cases have emerged at a warehouse in Bologna used by express courier Bartolini, including 17 friends and relatives of workers.

Italian media identified 10 cases of new clusters across the country this week, including in care homes in Como and Alessandria in the north, and a religious institute in Rome.