Rescuers on Wednesday searched for four workers still missing after an explosion at a hydroelectric plant in Italy, the fire service said, revising the death toll from four to three.

A local authority official in nearby Bologna had on Tuesday evening told AFP that four people were killed in the incident at the Bargi hydroelectric plant run by Enel Green Power on Lake Suviana, near Bologna.

But this was revised overnight.

"The toll is four missing and three dead," the fire service spokesman told AFP on Wednesday morning.

"The searches are continuing, the situation is particularly complex," with water still entering the plant, he added.

Five people were injured, according to the AGI news agency, which named the dead as three men aged 73, 45 and 35.

The mayor of the nearby town of Camugnano, Marco Masinara, called the explosion a "terrible workplace accident" that affected the "entire community".

"It seems there was a floor slab collapse and rescue is difficult as a lot of water entered inside the eighth basement floor," he said late on Tuesday.

Enel Green Power, the renewables unit of energy giant Enel that operates the plant, has said it was coordinating with authorities. 

In a statement to AFP on Tuesday evening, it said "a fire affected one of the two groups of the Bargi plant" in the province of Bologna.

"Following investigations, the dam basin of the Bargi plant was not damaged and is safe," it said.

Production was halted, but there was no impact on local or national energy supplies, it said.