The highest temperature of the year was logged Wednesday as a protracted heat wave continues to sear wide swathes of Japan, leading the government to issue a warning to the public to protect themselves against heat-linked illnesses.

The Fire and Disaster Management Agency said that 12 people have died in the heat wave during the week through last Sunday and almost 10,000 people were hospitalized for heat-related illnesses, such as heatstroke and dehydration.

Of those hospitalized, the majority were in Osaka Prefecture, with 752 people being sent to hospital, followed by Tokyo with 704 people and 687 people in Aichi Prefecture, local media reported.

The figure for those sent to hospital for treatment, the agency said, was 3.7 times higher than that of the previous week and, according to local media, around 5,000 people alone were hospitalized over the three-day holiday weekend.

Temperatures on Wednesday hit 40.7 degrees Celsius in Tajimi City in Gifu Prefecture, central Japan, marking this year's highest temperature, with the mercury also soaring to 40.6 degrees Celsius in Gifu's Mino City.

Japan Meteorological Agency (JMA) said that of the 927 points it monitors across the nation, 185 of them saw temperatures rise to 35 degrees Celsius or more, with the heatwave expected to continue for a week.

The weather agency has forecast daytime highs of at least 35 degrees Celsius for Nagoya, Kyoto, and numerous other regions over the next seven days.

Of the heat-related fatalities, in Toyota City, Aichi Prefecture, a six-year-old boy died of heatstroke on Tuesday after a school trip outdoors where the temperature was about 33 degrees Celsius, local media reported.

A 94-year-old woman is also believed to have died from heatstroke in Nabari City, Mie Prefecture. She was found by her relatives on Tuesday evening lying in a field unconscious, according to local media accounts.

As for the capital city of Tokyo, a record 2,900 emergency calls were received by the Tokyo Fire Department on Tuesday, marking the highest number of calls received in a single day since emergency services began in 1936.

The JMA has said that the heat wave will continue throughout the week ahead owing to powerful convection activity near the Philippines, where air currents are strengthening a Pacific high-pressure system.

The protracted heat wave has made it particularly hard for ongoing relief operations in areas in western Japan pummeled by recent torrential rains.

In Okayama, Hiroshima and Ehime prefectures, the three hardest-hit regions by the downpours, 813 people were reportedly hospitalized with heat-related illnesses last week, including volunteers who were cleaning up debris and assisting at evacuation shelters.