A city in eastern Japan logged the country's highest temperature for a June day on Saturday, rising above 40 Celsius, the Meteorological Agency said.

The heat wave, which the agency expects to persist throughout the summer, came as the government called on households and businesses to save electricity to avoid a possible power crunch until September.

A weather station recorded a temperature of 40.2 Celsius (104.36 Fahrenheit) on Saturday afternoon, in Isesaki, Gunma Prefecture, about 85 kilometers (53 miles) northwest of Tokyo, according to the agency.

The figure broke the previous record of Japan's hottest June day — 39.8 C observed on June 24, 2011.

"A strong anticyclone from the Pacific, coupled with a cloudless weather, brought the heat," a JMA official said.

Another weather station in central Tokyo logged 35.4 C earlier in the day, marking the earliest arrival since records began in 1875 of an above-35 C day in the capital, considered extreme heat in Japan.

On Saturday, agency and the environment ministry issued a heatstroke alert in six of the country's 47 prefectures, recommending people to remain indoors and use air conditioners.

In its latest three-month weather forecast released this week, the agency said this summer will be hotter than regular years in northern, eastern and western Japan, due to factors such as the global warming and La Nina.

The forecast adds a concern for the country, which faces tighter energy supply due to the slow restart of nuclear power, thermal plant shutdowns and geopolitical risks heightened after Russia's invasion of Ukraine.

Temperatures were already high in other parts of Japan on Friday, with the city of Nagano registering 35.1 C just after noon and 36.7 C in Niigata Prefecture’s Takada district, the agency said earlier.

In the week through Sunday, 1,337 people nationwide were sent to the hospital due to suspected heatstroke, according to the Fire and Disaster Management Agency.

The meteorological agency’s two-week forecast for the Kanto-Koshin region shows sunny weather and high temperatures throughout next week, prompting some meteorologists to speculate that the rainy season could end exceptionally early this year.

Typically, the rainy season starts in early June and lasts until mid-July on the mainland. The earliest recorded end of the rainy season in the Kanto-Koshin region was June 29 in 2018. Last year, the rainy season in the region ran from June 14 to July 16, starting later than usual.