The west of Japan had its hottest ever summer this year, official figures showed Monday after a season in which heatstroke reportedly killed hundreds and hospitalised tens of thousands nationwide.

The average temperature from June to August was 1.2 degrees Celsius higher than the seasonal norm, with the mercury hitting a record 41 degrees C (105.8 Fahrenheit) in the western city of Kochi on August 12, according to the Japan Meteorological Agency.

That broke the old high of 40.9 degrees Celsius registered in two central cities in August 2007.

One week in August saw temperatures of 40 degrees or more for three straight days in parts of Japan.

Of the nearly 54,000 taken to hospital suffering from heatstroke over the three months from late May, 87 people died, the Fire and Disaster Management Agency said.

Public broadcaster NHK, which did its own tally to include those found dead at home, said at least 338 died of heatstroke between late May and August, nearly 80 percent of whom were in their 60s or over.

Japan's west covers the business hub of Osaka and the ancient city of Kyoto as well as Kyushu and Shikoku, the country's third- and fourth-largest islands.

Eastern Japan also had a hot summer, with average temperatures 1.1 degrees C higher than normal, making it the third hottest on record, the weather agency said.

The hot summer followed an early end to the rainy season, sparking fears over dwindling water supplies that saw scientists resort to cloud-seeding to top up reservoirs that serve the 35 million people of greater Tokyo.

However, in western Japan a number of days of very heavy rainfall caused problems, with landslides and flooding in places.

Yamaguchi prefecture in the west of Honshu island recorded 14.3 centimetres (5.6 inches) of rain in an hour on one occasion in July.