An intense heatwave is expected to persist across much of Japan. Authorities are warning of dangerously high temperatures in some areas.

The Meteorological Agency says the mercury on Saturday will approach 40 degrees Celsius mainly in the east and west.

Daytime highs are forecast to reach 39 degrees in Hita City in Oita Prefecture, and Kyoto City. The temperature could reach 38 degrees in the cities of Nagoya, Gifu, Nara, Okayama and Kumamoto.

The agency and the Environment Ministry have issued a heatstroke alert for 37 of the country's 47 prefectures.

People are urged not to exercise and to avoid going outside as much as possible.

They are advised to use air conditioning indoors and to regularly consume adequate amounts of fluids and salt.

People are also asked to keep a close eye on infants and the elderly, who are at higher risk of developing heatstroke.

Authorities say atmospheric conditions will become unstable from Saturday afternoon. They are warning of possible localized torrential rain with lightning across wide regions from eastern Japan down to Okinawa and Amami in the southwest.

People are also advised to be on the alert for strong winds and hail.