Four people died in Spain, two in France and two in Italy as an early summer heatwave continued to grip much of Europe on Wednesday, triggering health alerts and forest fires and forcing the closure of a nuclear reactor at a Swiss power plant.

Spanish officials said a wildfire in Catalonia had killed two people a day earlier, and authorities reported heatwave-linked deaths also in Extremadura and Cordoba. France's energy minister reported two deaths linked to the heat, with 300 others taken to hospital.

Italy issued red alerts for 18 cities, while in Germany temperatures were forecast to peak at 40 degrees Celsius (104 degrees Fahrenheit) in some areas, making it the hottest day of the year.

Two men over the age of 60 died in separate incidents on the beach in Sardinia from the heat, ANSA news agency reported.

Weather forecaster Meteo France said red alerts remained for several areas of central France.

The risks were greatest for vulnerable members of the population, and Catherine Vautrin, France's health and families minister, said authorities should remain vigilant.

"In the coming days, we'll see the consequences, particularly on the most vulnerable, and I'm thinking particularly of the elderly," she said.

Turkey, which battled blazes on several fronts forcing the temporary evacuation of about 50,000 people earlier in the week, said its fires were largely contained.

Tuesday's fire in the Catalonia region of Spain destroyed several farms and affected an area stretching about 40 km (25 miles) before being contained, officials said.

STORMS AHEAD, REACTOR SHUT

Italy, France and Germany warned of the risk of heavy storms due to excessive warming in unstable atmospheres. Violent storms in the French Alps late on Monday triggered mudslides, disrupting rail traffic between Paris and Milan.

Swiss utility Axpo shut down one reactor unit at the Beznau nuclear power plant and halved output at another on Tuesday because of the high temperature of river water.