South Korea confirmed 813 more coronavirus cases on Saturday, the biggest increase to date for the country and taking the national total to 3,150 infections with four additional deaths.
Authorities also reported the country's first case of reinfection -- a 73-year-old woman who tested positive for a second time after her recovery and release from hospital last week.
The illness recurred "as her immune system had declined", said Korea Centers for Disease Control and Prevention (KCDC) official Kwon Jun-wook.
At least 10 reinfection cases have already been reported in China, Kwon added.
More than 90 per cent of the new cases were in Daegu, the center of the country's outbreak, and its neighbouring North Gyeongsang province, the Korea Centers for Disease Control and Prevention (KCDC) said in a statement.
Three women in the Daegu area and one man died of the illness, taking the national toll to 17, the statement added. All were aged in their 60s or older.
The national total is expected to rise further with a screening of more than 260,000 people associated with the Shincheonji Church of Jesus, a secretive entity often accused of being a cult that is linked to around half of the country's cases.
A 61-year-old female member developed a fever on February 10 but attended at least four church services in Daegu -- the country's fourth-largest city with a population of 2.5 million -- before being diagnosed.
Facing public criticism, a Shincheonji spokesman said the church members were victims of a "witch hunt".
"I ask you to discard hatred and criticism against our members," said the spokesman in a video posted on the Shincheonji website.
More than 85 per cent of South Korea's coronavirus cases are in Daegu and the neighbouring province of North Gyeongsang, according to the KCDC.
Of those who test positive for the virus, 80 per cent can be treated with medication because they have "light symptoms", said vice health minister Kim Gang-lip, adding that the rest would need more advanced treatment in hospitals.
The streets of Daegu have been largely deserted for days, apart from long queues at the few shops with masks for sale.
Authorities have urged the public to exercise caution and anyone with a fever or respiratory symptoms to stay home.
But officials say they are not considering a citywide quarantine similar to the lockdown imposed on the central Chinese city of Wuhan, where the virus first emerged.
A surge in confirmed cases has led many events to be cancelled or postponed as the outbreak has hit the world's 12th-largest economy, including concerts by K-pop superstars BTS and the World Team Table Tennis Championships.
The new school term has been delayed by one week nationwide while schools in Daegu by three weeks, and the US and South Korean militaries have postponed forthcoming joint exercises.
Auto giant Hyundai Motor also suspended operations at one of its Ulsan plants after an employee tested positive for the coronavirus.