South Korea has reported its highest ever one-day rise in new Covid-19 cases, as officials consider imposing a semi-lockdown in the capital Seoul, the epicentre of the country's recent rise in infections.

The 1,275 cases in the 24 hours to midnight on Wednesday, reported by the Korea Disease Control and Prevention Agency (KDCA), exceeds the number of daily cases reported at the peak of the country's third wave in December.

Around 80 per cent of the 1,275 locally transmitted cases were recorded in the greater Seoul area, comprising the capital, Gyeonggi province and Incheon city, KDCA data showed.

Health authorities said Seoul would meet the criteria for imposing level four social distancing measures with a three-day streak of new case numbers above 389, the daily average from July 1-8.