SEOUL, South Korea — South Korea has reported 686 new cases of the coronavirus, tying its second-highest daily jump since the emergence of the pandemic, as a resurgence driven by the greater capital area threatens to erase hard-won gains against the virus.

The Korea Disease Control and Prevention Agency said Wednesday that 536 of the new cases were reported from the Seoul metropolitan area, where new clusters of infections have been popping up from seemingly everywhere, including restaurants, markets, saunas, hospitals, long-term care facilities and army units.

The country’s caseload is now at 39,432, including 556 deaths. The agency said 149 among 8,699 active patients were in serious or critical condition, a group that is being closely monitored amid concerns about a possible shortage in intensive-care beds.

South Korea had also reported 686 cases on March 2 during a major outbreak in its southeastern region, which health workers managed to contain by April with an aggressive test-and-quarantine campaign.

While South Korea had been seen as a success story against COVID-19 since, critics said the country let its guard down by easing social distancing restrictions to the lowest tier in October, even as experts warned of a surge during cold weather months when people spend longer hours indoors.