SEOUL, South Korea — South Korea has reported 79 new cases of COVID-19, its biggest daily jump in more than 50 days, a huge setback that threatens to erase some of the country’s hard-won gains against the virus.

The Korea Centers for Disease Control and Prevention on Thursday said 67 of the new cases were reported from the densely populated Seoul metropolitan area, where about half of South Korea’s 51 million people live. The national totals were at 11,344 cases and 269 deaths.

The country had reported 40 new cases on Wednesday, which was its highest figure in 49 days.

Health Minister Park Neung-hoo during a virus meeting on Thursday pleaded all residents in the greater capital area to avoid unnecessary gatherings and urged companies keep sick employees off work.

At least 69 infections so far have been linked to workers at a massive warehouse operated by local e-commerce giant Coupang. Health authorities say the company likely didn’t enforce preventive measures such as masks at the facility in Bucheon, near Seoul, and may have had employees working even when sick.

Hundreds of other infections have been linked to nightclubs and other entertainment venues, which saw huge crowds in early May after officials relaxed social distancing guidelines.

It remains to be seen whether the recent spike in infections force officials to push back a phased reopening of schools, which they had touted as a major accomplishment in the nation’s anti-virus campaign. The Education Ministry on Wednesday said class openings were delayed at 561 schools nationwide because of virus concerns.