Overnight temperatures in Seoul have sizzled above 25C for 22 consecutive days in July, officials said Thursday, marking the longest such streak for the month since modern weather records began in October 1907.

Such evenings are known as "tropical nights" in South Korea.

The capital was also on track to record its hottest July night in history on Wednesday, with the lowest temperature of the day reaching 29.3C -- unless overnight temperatures rise even higher on Thursday.

Much of the world is enduring a summer of sweltering weather, with June being the hottest month ever recorded for 12 countries, according to an AFP analysis of data from the European climate monitor Copernicus.

The intense heat in Seoul is expected to continue, according to the meteorology office.

"The warm air from the North Pacific High began affecting South Korea a bit earlier than usual," Youn Ki-han, director at Seoul's Meteorology Forecast Division, told AFP.

"As that influence gradually built up, the heat accumulated, much like interest compounding in a savings account, slowly increasing over time," said Youn.

"Normally, if it's hot for just a day, temperatures spike and then quickly return to normal. But when warm conditions persist for several days, the heat doesn't fully dissipate, it lingers and accumulates with each day," he added.

Thirteen people have died from suspected heat-related causes so far this year, triple the number from the same period last year, according to data from the Korea Disease Control and Prevention Agency.

Hundreds of thousands of livestock have also perished amid the extreme heat.