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.