A powerful tornado on Wednesday struck a steel plant in southern Italy at the centre of a labour dispute, leaving 20 workers slightly injured and one missing, as storms lashed much of the country.

The missing worker at the ILVA plant in Taranto is believed to have been operating a crane at the port which was swept out to sea by the wind.

"Divers are searching for him," Lino Ursi, a spokesman for Taranto's mayor, told AFP.

A chimneystack was partly destroyed by the strong winds and explosions could be heard inside the closed facility, officials said.

A regional emergency official, Fabiano Amati, said the storm had also struck a nearby school and six children had been lightly injured.

The ILVA plant, which is the biggest steel mill in western Europe, has been shut since Monday because of a long-running dispute over pollution levels.

The company said there had been "heavy structural damage" on the facility.

Four people were also killed in a car crash in the southern Apulia region where Taranto is located, which officials blamed on the stormy weather.

Tuscany was also affected for the second time in two weeks, with streets flooded in many areas including Florence and the province of Grosseto.

Ferries were suspended in the Gulf of Naples, emergency services were on the alert in the Emilia-Romagna region struck by twin earthquakes earlier this year and rising waters also flooded parts of the historic heart of Venice.