ROME, June 28 (Xinhua) -- Italy's first heatwave of the summer is set to intensify this weekend, with temperatures expected to reach 40 degrees Celsius in several regions, according to Italian authorities.

The Health Ministry has issued red alerts for 12 major cities, including Rome, Milan and Turin, for Friday and the following weekend. The number of cities under red alert is expected to rise to 17 by Saturday, and to 21 on Sunday.

The heatwave comes as average temperatures in Italy have risen to 16.9 degrees Celsius, compared to 14.6 degrees Celsius just 50 years ago, according to a report from Il Meteo, Italy's weather monitoring site.

Lorenzo Tedici, a meteorologist with Il Meteo, attributes the rise to higher sea temperatures, a key factor for the peninsula.

"Until recent years, the sea acted as a natural temperature regulator," Tedici explained. "Now, it has become an amplifier of the heat." Enditem