A heatwave warning was in place and the Seoul city government on Wednesday said the extreme temperatures were a "disaster" that posed a threat to people's lives.

More than 1,200 people in South Korea have suffered heat-related illnesses since May 15, with eight deaths reported, according to official data released Wednesday.

The figures mark a sharp rise from the same period last year, when 486 cases and three deaths were recorded, the data showed.

On Tuesday alone, 238 people went to the emergency room nationwide after suffering heat-related conditions, the data indicated.

Seoul said it would "prioritise preventing casualties through tougher inspections and enhanced measures".

According to the national weather agency, Tuesday hit 37.8 degrees Celsius, marking the hottest early July day - defined as July 1 to 10 - in the South Korean capital since comprehensive records began 117 years ago.

South Korea experienced last year its highest average summertime temperature since such records began half a century ago -- nearly two degrees higher than the historic average, according to the weather agency.

Earlier this year, the South was hit by the country's deadliest wildfires in history, which were fanned by high winds and ultra-dry conditions.

The country has also been grappling with the invasion of a type of March fly nicknamed "lovebug" -- which leave behind piles of rotting black remains and a foul stench after they quickly die -- that experts say highlights worsening climate change.

Young climate activists scored a resounding victory last year when South Korea's Constitutional Court ruled that much of the country's climate goals were unconstitutional, because they did not go far enough to protect young people's futures.

The government was forced to revise its climate targets.

In Europe, scientists said Wednesday human-caused climate change made recent European heatwaves up to 4C hotter in many cities, pushing temperatures into deadly territory for thousands of vulnerable people.