An 80-year-old woman was found dead in the lavatory of her apartment in Saitama, north of Tokyo, apparently due to heat stroke, local medical authorities said.

In Sakai, a city by the Sea of Japan (East Sea) some 600 kilometres (375 miles) west of Tokyo, the mercury soared to 38.1 degrees Celsius (100.58 F), one of 21 local records, the Japan Meteorological Agency said.

The highest temperature in the nation Tuesday was 38.6 C (101.48 F) in nearby Yonago, although it was a notch below the town's record high, the agency said.

Japan has a mostly temperate climate with strong seasonal variations. The country's all-time heat record was 40.8 C (105.44) registered in July 1933 in landlocked Yamagata, some 300 kilometres (190 miles) north of Tokyo.

The mercury also soared high Tuesday at the peak of Mount Fuji, Japan's highest mountain at 3,776 metres (12,388 feet).

Its reading reached an unprecedented 18.2 C (64.76 F) but the agency decided against recognising it as an official record because it was measured only by auxiliary observation equipment at an unmanned monitoring post.

The agency forecast the heat wave to prevail for the rest of the week as high air pressure stays strong due to hot temperatures on the surface of the northwestern Pacific Ocean.