A child runs through a fountain on Gwanghwamun Square in Seoul on Wednesday, amid an unrelenting heat wave across the nation. (Yonhap)

A heat wave watch was issued for all parts of Seoul on Sunday, the first time it was issued this late in the year, according to the weather authorities.

A heat wave watch is issued when the highest sensible temperature is expected to reach 33 C or higher for two or more consecutive days or when major damage is anticipated from the heat.

Other parts of the country, meanwhile, including Gongju and Cheongyang in South Chungcheong Province, and Yongin and Anseong in Gyeonggi Province, were issued heat wave warnings, which indicate the highest sensible temperature reached 35 C or higher for two or more consecutive days or major damage was anticipated from the heat across a wide area.

Tropical nights were recorded overnight in areas as wide-ranging as Incheon and Daejeon, Mokpo, Busan, Pohang and Jeju.

The number of tropical nights, or nights when the lowest temperature is 25 C or higher, increased to 69 in Jeju this year.

On Sunday, temperatures are expected to reach highs of 23 to 36 C across the nation, with the mercury rising to 28 C in the southeastern city of Ulsan, 31 C in Seoul, Incheon, Daegu and Busan, and 34 C in Daejeon and Gwangju.