MILAN - Fire brigades in Tuscany battled a wildfire on Wednesday that forced hundreds to evacuate and caused gas tanks to explode, as smoke from a blaze in northeast Italy forced shipbuilder Fincantieri to shut down a 3,000-staff plant.

Wildfires have broken out in several parts of Italy this week as temperatures keep rising.

Nine cities were on the country's highest heatwave alert, which warns of serious health risks linked to the weather, up from five on Tuesday. The total will rise to 14 on Thursday, including Rome, Milan and Florence.

Temperatures are expected to hit 40C across a swathe of the north and centre this week, as well as in the southern heel of Italy's boot, Puglia, and the islands of Sardinia and Sicily.

On Wednesday, in Tuscany, a fire that broke out on Monday evening continued to burn near the town of Lucca, having already destroyed some 560 hectares (2.15 square miles) of woods.

It forced around 500 people to evacuate as the flames raged through the night reaching some villages and causing the explosion of some liquified gas tanks, the region's governor Eugenio Giani said on Twitter. "Some fronts have strengthened because of the wind," he added.

In the northeastern Friuli Venezia Giulia region, residents were urged to stay indoors because of heavy smoke from a wildfire that started on Tuesday in the Carso area bordering Croatia and Slovenia.

The fire prompted state-owned shipbuilder Fincantieri to close down its plant in the port city of Monfalcone.

(Reporting by Valentina Za and Elvira Pollina; editing by John Stonestreet)