Some areas in central Japan registered record high temperatures of 40C, said the Japan Meteorological Agency. Yonhap news agency said that the temperature in Gangneung, less than 170 km east of capital Seoul, recorded the highest-ever morning low of 31 degrees Celsius at 6.45 am, reported AP.
The heat, due to a layering of two high pressure systems over much of Japan, is expected to ease slightly this week but temperatures of around 33 deg C are expected.
A total of 3,091 ambulances were dispatched in Tokyo on Saturday, a record for a single day, it said.
The mercury hit 39.9 degree Celsius in the southeastern town of Hayang, the highest recorded in the country so far this year.
People across the country have been urged to take precaution as thousands have sought hospital treatment for heat-related conditions over the past two weeks. Numerous victims have been elderly people who were not using air conditioning.
There are still about 4,500 people living in emergency shelters in the wake of the flooding and Japanese authorities were considering housing the displaced people on a cruise ship, according to the Japan Times.
The heatwave has also raised questions over the 2020 Olympics, set to take place in Tokyo over July and August.
"For the athletes, they're trained but for spectators who are cheering on the road, we may not necessarily be able to say the same", she said at a Monday press conference.
In North Korea, residents fanned themselves on crowded trolleys or protected themselves from the sun with parasols as temperatures in Pyongyang, the capital, reached 34C.
The marathon and some other outdoor Olympic events will start early in the morning.
Koike also cited traditional ways of cooling in Japan, such as hanging straw screens and spraying water on road surfaces.
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