South Korea's two largest cities of Seoul and Busan have shattered records for the longest streaks of tropical nights as a relentless heat wave grips the nation.

Seoul experienced its 27 consecutive tropical night, with the lowest temperature of 27.2 degrees Celsius recorded between Friday night and early Saturday morning, according to the Korea Meteorological Administration.

This extended the record for the longest number of tropical nights in a row since modern meteorological observations began in the capital city in 1907. The latest streak began July 21.

A tropical night occurs when temperatures remain at or above 25 degrees Celsius from 6:01 p.m. until 9 a.m. the following day.

The southeastern port city of Busan also recorded its 23rd consecutive tropical night, with the lowest temperature of 26.6 degrees Celsius. The streak, which began on July 25, is the longest since the modern weather monitoring started in the city in 1904.

Incheon experienced its 25 consecutive tropical night, the second-longest streak for the city west of Seoul, following a 26-day run in 2018.

On the southern island of Jeju, tropical nights continued for the 33rd day in a row. (Yonhap)