South Korea registered on Sunday its highest temperature since records began more than a century ago, according to the state weather agency.

The southeastern city of Yangsan saw the temperature reach 42.5 degrees Celsius at 1:26pm (9:26am PKT), the Korea Meteorological Administration said on its website.

Yangsan has been the epicentre of the ongoing heatwave, where temperatures exceeded 40°C for a fifth consecutive day on Sunday.

Much of South Korea has been placed under heat warnings, with many cities seeing temperatures climb into the high 30°C.

More than 20 localities were on Sunday under emergency heatwave alerts, a new warning category introduced this year to better address rising temperatures.

An emergency alert is issued when areas experiencing a heatwave are forecast to hit perceived temperatures of 38°C or an actual temperature of 39°C for one day.

KMA data show the average annual number of heatwave days in the country has more than doubled to 19 over the past five years, from eight in the 1970s.

A heatwave day is defined as one with a maximum temperature of at least 33°C, while a tropical night is one when the overnight low remains 25°C or above.

Scientists warn that extreme weather events such as heatwaves are becoming more frequent and intense as a result of human-induced climate change.