A heatwave stifled Japan Sunday as the temperature topped 40 degrees Celsius (104 Fahrenheit ) in two cities, leaving at least four people dead over the weekend, officials and reports said.

The Japan Meteorological Agency said the temperature reached 40.6 C in Kofu, 100 kilometres (60 miles) west of Tokyo, in mid-afternoon.

The weather agency had warned early Sunday that the temperature would soar past 35 C in 39 of the country's 47 prefectures. It warned people to drink plenty of water and use air-conditioners.

On Saturday the mercury topped 40 C for the first time in Japan since August 2007, when it had reached an all-time high of 40.9 C in two separate cities.

An 84-year-old woman and a 66-year-old man in western Japan died from heat stroke on Saturday after they were found collapsed in fields, Kyodo news agency said.

Two more deaths from heat stroke were confirmed by officials on Sunday.

An 80-year-old woman died in hospital after she was found collapsed at her wooden home Sunday morning in Arita, a city south of Osaka, a medical evacuation official said. The living room where she was found was not air-conditioned.

In Saitama north of Tokyo, a 60-year-old man died in hospital after he was found unconscious on a street Saturday afternoon.

The heatwave, also gripping parts of China, has been caused by a Pacific high pressure system covering most of the Japanese islands, the weather agency said.

In early July a heatwave in the country claimed at least a dozen lives, according to media reports.