Japanese officials issued new warnings Monday as a deadly heatwave blankets the country, producing record high temperatures in Tokyo just two years before the city hosts the 2020 Summer Olympics.

Officials said last week that the heatwave had killed at least 15 people and forced the hospitalisation of over 12,000 others in the first two weeks of July.

But the death toll may be more than double that, with Kyodo News agency reporting 11 people died on Saturday alone across Japan.

An updated official toll is expected later this week.

The heatwave has toppled temperature records across the country, with Kumagaya in Saitama Prefecture setting a new nationwide record on Monday with temperatures hitting 41.1 Celsius (106 degrees Fahrenheit).

And in western Tokyo's Ome, temperatures hit 40.3 degrees Celsius, the first time temperatures over 40 have been recorded in Tokyo's metro area.

Records fell at 13 other observation stations across the country, with more than a dozen cities and towns seeing temperatures around 40, the Japan Meteorological Agency said.

Japan's summers are notoriously hot and humid, and hundreds of people die each year from heatstroke, particularly the elderly in the country's ageing society.

But this year's record temperatures have surprised residents and officials alike, and revived concerns about the 2020 Summer Olympics, which will be held in July and August in Tokyo.