The Franco-Italian carmaker assembles the recently launched Alfa Romeo Tonale SUV, production of which is ramping up, and the Fiat Panda small car at the facility in Pomigliano, near Naples.

The plant employs over 4,000 people, with over 20% of them still away on state-funded furloughs.

Mario Di Costanzo, the head of leftist metalworker union FIOM's branch in Pomigliano, told Reuters that production halted almost entirely on Wednesday and suffered further stoppages for several hours on Thursday and on Friday.

A spokesman for Stellantis in Italy told Reuters that the strikes never halted production lines in Pomigliano completely but that output was slowed down.

Fiom's Di Costanzo said the company was currently raising vehicle production to respond to increased market demand without adding staff to assembly lines, instead pressing workers to increase productivity during their shifts.

"The company is seeking effective cost reductions," Di Costanzo said. "This means backbreaking shifts and fewer investments on the lines and on workplace conditions, including safety."

He added cost cuts were also affecting several other services within the facility, including cleaning.

The Stellantis spokesman said the company rejected any accusation of a lack of attention to the working conditions of its employees.

"Stellantis always acts within the framework of the specific labor contract and with respect for its employees, also considering the competitive environment in the automotive industry," he said.

Stellantis agreed a pay deal with Italian unions in March that will raise wages by 11% over the first two years but FIOM was not part of that settlement.

(Reporting by Giulio Piovaccari; Editing by Keith Weir)

By Giulio Piovaccari