A lingering high pressure system has brought record-high temperatures to the region for more than a week, killing more than 40 people in Japan and 10 in South Korea.
Temperatures peaked at 40C in central Japan earlier this week, a five-year high. Media reports say at least 11 more died this past Saturday alone, while thousands have been taken to hospital.
More than 9,900 people were rushed to hospital during the week to mid-July, jumping from 2,700 in the previous week, the Fire and Disaster Management Agency said.
Among the dead was a primary school boy who collapsed after a field trip to a park 20 minutes' walk from his school.
An updated official toll is expected later this week.
The heat wave has prompted the agency to issue warnings for high temperatures across much of the nation. 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.
Children play in the water jets at a park near Nerima in Tokyo as temperature rose on Monday
"And at even lower temperatures, the heat can be risky for small children and elderly people, and depending on the environment and activities you are doing", she warned.
And people have been warned to brace for more scorching weather that threatens to last until the end of the month.
Japan's meteorological agency urged people to drink sufficient water to prevent heat exhaustion.
About 200 people were killed in the devastating floods and landslides, with the Japanese government setting aside 70 billion yen ($US631 million) to assist with the recovery.
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.
The heatwave has also raised questions over the 2020 Olympics, set to take place in Tokyo over July and August.
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An ambulance arrived at the scene in eight minutes and crew assessed the boy's injuries. "We have no update on his condition". Anyone with information that could help police with their enquiries should call 999 quoting incident 442s of July 21, 2018.
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