The scorching heat is estimated to have killed 23 people across the country, more than triple the record of seven during the same period last year, local media reported citing firefighting authorities.

The official temperature, measured in the city of Yeoju, south of Seoul, hit 38.4C on Tuesday, the Ministry of the Interior and Safety said.

South Korea raised the heat warning level in its four-tier system to the highest as of 6 p.m. on Tuesday, the first time since 2019.

The highest "serious" warning is issued when the apparent temperature is expected at 35C or higher in at least 40% of the country's 180 regions for three or more days. It can also be issued when the apparent temperature is likely to be 38C or higher for three or more days in 10% of the country.

The highest daily temperature on Wednesday is expected to reach 35C in the capital Seoul, the weather agency said.

The government said it expected high temperatures to persist with oppressive humidity for the coming days, with the apparent temperature likely to hover around 35C in most parts of the country.

President Yoon Suk Yeol on Tuesday urged officials to step up measures to prevent further casualties, especially for people working outdoors, the elderly citizens and those living in makeshift houses without adequate air conditioning systems.

Near Yoon's office on Wednesday, construction workers held a press conference and called for concrete countermeasures.

"Under the current conditions, construction workers' heat deaths are 'expected deaths,'" the Korean Confederation of Trade Unions said in a statement.

The growing frequency and intensity of severe weather is symptomatic of global, human-driven climate change, experts in the field say, with heatwaves in much of the world expected to persist through August.

North Korea is also battling the unrelenting heatwave with highest daily temperatures forecast to hover around 35C to 37C by Thursday, state media reported.