Japan has experienced its first "kokushobi", or "cruelly hot day", as temperatures in parts of the country climbed past 40C (104F).

The term was introduced in April by the country's weather agency to describe days when temperatures reach 40C, with the aim of more effectively warning the public about dangerous heat.

On Tuesday, temperatures in three central Japanese cities - Tajimi, Gujo, and Toyoda - hit that threshold. The heat is expected to continue into next week, said Japan's Meteorological Agency (JMA).

It comes as extreme weather events like heatwaves are becoming more intense around the world. Last year, Japan saw its highest-ever temperature of 41.8C in the city of Isesaki.

Japan currently already has terms for days over 25C, 30C and 35C.

Days with temperatures exceeding 35C are known as moshobi - or "extremely hot days".

But its more severe counterpart uses the term koku - meaning harsh or cruel - to describe the heat.

Nearly 300 out of the country's 914 observation points recorded a moshobi on Tuesday, Kyodo News reported.

It also comes as authorities have issued heatstroke alerts in more than 40 prefectures across the country. They have also advised people to use air-conditioners, drink enough water and consume salt, as well as avoid prolonged outdoor activities.

The Tokyo Fire Department said a total of 287 people, aged three to 103, were taken to the city's hospitals on Tuesday because of apparent heatstroke - the highest number recorded this year, said national broadcaster NHK.

The fire department also issued an ambulance overload alert on Wednesday and urged people to protect themselves from the heat.

It comes as Japan's summers are getting hotter and more dangerous. Last year, the number of people who suffered heatstrokes at their workplaces reached a record 1,803, said a report by news outlet Kyodo. Nineteen of them died.

As Japan braces for another hot summer, the Tokyo government has encouraged office workers to adopt more casual attire like T-shirts and shorts, instead of suit and ties, because of the heat.