South Korea reported its biggest jump in coronavirus infections in seven weeks on Wednesday, driven by a fresh cluster at an e-commerce warehouse on Seoul's outskirts, as millions more pupils went back to school.
The country has been held up as a global model in how to curb the virus and has rushed to contain new infections as life returns to normal.
But officials announced 40 new cases Wednesday -- taking its total to 11,265 -- with most new infections from the densely populated Seoul metropolitan area.
It was the largest increase since 53 infections were announced on April 8.
An outbreak at a warehouse of e-commerce firm Coupang in Bucheon, west of Seoul, has seen 36 cases so far, the KCDC added.
"It is suspected that the basic regulations were not enforced at the warehouse," said vice health minister Kim Gang-lip.
"If quarantine rules are not implemented at workplaces, it could lead to a dreadful result of a mass infection."
But officials said the possibility of parcel recipients being infected was low.
Social distancing rules have been relaxed in South Korea and facilities such as museums and churches have reopened while some professional sports -- including baseball and soccer -- started new seasons earlier this month, albeit behind closed doors.
More than two million students returned to classes on Wednesday, as part of the phased reopening of schools.
Some high schools were forced to turn back pupils shortly after resuming classes last week over concerns of new virus cases in their neighbourhood.
The country endured one of the worst early outbreaks of the disease outside mainland China, and while it never imposed a compulsory lockdown, strict social distancing had been widely observed since March.
But it appears to have brought its outbreak under control thanks to an extensive "trace, test and treat" programme.