Health officials say they'll consider tougher social-distancing measures if the current uptick continues.
There have been 79 new cases reported in the last 24 hours, the highest daily rate of new infections in nearly two months.
Most have been traced to two warehouses on the edge of the capital, Seoul. The centres are owned by South Korea’s biggest e-commerce firm, Coupang.
Nearly 4,000 workers from the distribution centres have been tested and are in self-isolation.
More than 560 schools across the country have closed their doors and resumed online classes, just days after they reopened.
Social-distancing rules have been relaxed in South Korea in recent days, with museums and churches reopening.
South Korea has been widely praised for its aggressive track, trace and test approach to this pandemic which is credited with saving lives. Fewer than 270 people have died.