A weather forecaster looks at temperatures across South Korea at the Korea Meteorological Administration in Suwon, Gyeonggi Province, on Sunday, when heat wave alerts were issued in most parts of South Korea. (Yonhap)

Eleven people died of heat-related illnesses and four people drowned to death over the weekend in South Korea, authorities said Sunday.

The death toll from heat-related illnesses included mostly people in their 70s and older found dead inside greenhouses or on farms in regions where temperatures shot up to daily highs of between 33 and 36 degrees Celsius.

Seven people died due to the heat on Saturday alone, the Ministry of Interior and Safety said Sunday. Four more deaths were reported Sunday.

The dead included four people in their 70s through 90s in North Gyeongsang Province, two in South Gyeongsang Province, two in Gyeonggi Province and one in North Chungcheong Province.

The average age of the dead was 78.4, and their time of death was between 11 a.m. and 5 p.m.

A total of 1,015 people were reported to have suffered from heat-related illnesses between May 20 and July 29, and 178 of the cases were reported in the three days to Friday, according to the ministry.

Four people are reported to have drowned to death in the ocean and mountain streams over the weekend.

As demand for electricity spikes during periods of extreme heat, power outages also occurred at apartment complexes in Yongin, Gyeonggi Province, and the southwestern city of Gwangju.