Many places in Japan remain in the grip of sweltering heat, with temperatures reaching over 39 degrees Celsius in the southwestern region of Kyushu.

Weather officials say temperatures rose rapidly under sunny skies in eastern and western Japan on Tuesday.

They say daytime highs topped 35 degrees at more than 200 observation points from Kyushu to northeastern Japan's Tohoku region.

In Bungo-ohno City, Oita Prefecture in Kyushu, the temperature climbed to 39.2 degrees. Hamamatsu City in Shizuoka Prefecture, central Japan, saw the mercury rise to 38.9 degrees. Tokyo had a high of 36.1 degrees.

As of 6:00 p.m., temperatures were 30 degrees or higher in many areas.

Weather officials expect temperatures to remain above 25 degrees overnight.

They say the intense heat will continue into Wednesday, with highs of 38 degrees forecast for Kochi City in the Shikoku region, Kofu City in Yamanashi Prefecture and Hamamatsu, and 36 degrees in Tokyo.

The northern regions of Tohoku and Hokkaido are also likely to be affected by the severe heat wave.

The Meteorological Agency and the Environment Ministry have issued heatstroke alerts for 26 prefectures, including Tokyo, Osaka and Kagoshima.

Authorities are urging people to avoid nonessential outings, use air conditioners without hesitation and frequently take in fluids and salt.

They say special attention needs to be paid to the elderly and infants as they are at higher risk of developing heatstroke.