“All of Tuscany is united in grief for the tragedy that happened today,” Eugenio Giani wrote in a social media post.

The explosion occurred in Calenzano, on the northern outskirts of the Tuscan capital, in an area were trucks load fuel to be taken to petrol stations.

Nearby tanks at the depot were not affected.

“Otherwise the outcome would have been more tragic,” Giani told reporters.

Firefighters said the fire had been put out, while Italy’s civil protection department urged the local population to stay away from the accident and stay indoors.

The fire produced a thick column of black smoke, visible from a distance.

Experts from the region’s environment protection agency were on site to assess any potential environmental damage, including to waterways, Giani said.

The head of the CISL trade union, Luigi Sbarra, called for a thorough investigation and said “all companies, large and small, must comply with regulations on accident prevention” and invest more on maintenance, safety checks and workforce training.

“Protecting human lives comes before profit,” he added.

The prosecutor’s office in the nearby town of Prato said it would open an investigation to determine criminal responsibility for the incident, adding that the police, forensic doctors and technical advisers were already involved.

According to state agency Inail, more than 590,000 workers were injured in Italy last year, including 1,147 who lost their lives.

Better workplace safety was one of the demands behind a national strike held on Nov 29.