South Korea reported 79 new Covid-19 cases on Thursday, the highest rise in nearly eight weeks.
The spike in infections led to a return of tougher social distancing measures and the closure of 251 schools in Bucheon, west of Seoul, reports the BBC.
Another 56 new cases of coronavirus were recorded in the past 24 hours and the concern is that the infections are close to highly populated areas.
The spike in cases has been linked to a distribution centre in the area.
Health officials reportedly discovered traces of Covid-19 on workers' shoes and clothes at the warehouse which is run by South Korea's biggest e-commerce firm, Coupang.
Officials are tracking, tracing and testing thousands of employees from the warehouse and will continue to carry out further checks over the next two weeks.
“We’ll take every measure we can take until safety is confirmed,” a Coupang spokeswoman said.
More than 2 million children returned to class on Wednesday as part of a phased reopening of schools.
A student in Seoul, whose mother worked at the Coupang warehouse, was found to have Covid-19.
In addition to 251 schools in Bucheon closing after reopening, hundreds of others have delayed their reopening date.
The Education Ministry on Wednesday said class openings were delayed at 561 schools nationwide because of virus concerns.
South Korea was reporting around 500 new cases per day in early March before managing to stabilise its outbreak with aggressive tracking and testing, which allowed officials to relax social distancing guidelines.
But Seoul and nearby cities restored some of the controls in recent weeks by shutting thousands of bars, karaoke rooms and other entertainment venues to slow the spread of the virus.
Health authorities re-imposed some restrictions including the closure of places of worships, public parks and people are once again being told to avoid mass gatherings.
The country has now reported 11,344 cases and 269 deaths from Covid-19.
It comes as England is set to begin a phased return to school for some year groups on Monday, June 1.
Children in nursery, Reception and Year 1 and Year 6 pupils are due to return to the classroom.
The Government faced fierce opposition from teaching unions over safety concerns, with some councils saying they will delay the return to the classroom for their schools.
This week the Education Secretary Gavin Williamson warned schools must open on June 1 or risk closure for "months and months" as the Covid-19 outbreak could last for years.