South Korea's extended heatwave has taken its toll on both humans and animals, with more than 830,000 chickens or other poultry reported dead as of Wednesday.

The agriculture ministry said 786,512 chickens, 40,780 ducks, 3,000 quail, 336 pigs and five cows have died since July 20, when the peak temperature began hovering above 33 Celsius (91.4 Fahrenheit) in most areas.

The stifling heat also killed seven people in June and July, the health ministry said, mostly elderly people working in fields or greenhouses.

Temperatures have stayed above 35 C for 12 days in much of the country, causing massive blooms of algae in rivers.

Sales of electric fans and air conditioners have soared. On Monday the state power company warned that reserves were dangerously low and urged people to switch off appliances, as usage reached a record.

The meteorological administration says relief is in sight starting Friday, with midday temperatures dropping to the customary August figure of 30C from the weekend.