South Korea is on the brink of a nationwide COVID-19 outbreak and the number of daily new cases could further rise, health authorities said Sunday, as the country reported 397 new cases.

If the infection rate does not slow down this week, the authorities said they will consider raising the level of social distancing rules to the highest in the three-tier system, tantamount to a lockdown.

Of the new cases, 387 were locally transmitted and 10 were imported from overseas, with the total caseload at 17,399, according to the Korea Centers for Disease Control and Prevention.

Sunday marked the third consecutive day that Korea saw daily new cases above 300, with infections reported in all major cities and provinces except Jeju Island, Sejong and North Jeolla Province. The tally, which was up from 332 the previous day, was the biggest single-day increase since March 7 in the middle of the country’s first massive outbreak in spring.

“We don’t see it as a peak. We see that the number of cases can further rise,” the KCDC director Jung Eun-kyeong said at a briefing Sunday. Although contact-tracing is still underway, there could be many unidentified virus carriers out there and further chain transmissions are expected, she added.

The number of daily infections has been in the triple digits since Aug. 14 when 103 new cases were reported, mostly traced to a Seoul-based church at the center of the country’s resurgence of COVID-19. Over the past 10 days alone, 2,629 cases have been registered.

As health authorities saw the weekend as the last opportunity to contain a larger nationwide outbreak, they expanded tightened social distancing rules outside the greater Seoul area starting Sunday.

Under level two of the three-tier social distancing system, 12 types of high-risk businesses, such as nightclubs, karaoke rooms and Internet cafes, must be shut down. Gatherings of more than 50 people indoors and 100 people outdoors are banned.

All beaches around the country were closed starting midnight, weeks earlier than scheduled.

It will take more than one week for the heightened social distancing rules in place to draw a result, the KCDC director said, urging members of the public to minimize physical contact with others.

Mask-wearing was made mandatory in Seoul starting midnight Monday as the Seoul Metropolitan Government issued an administrative order to stem the spread of the virus. The capital is joining other provinces and cities including Gyeonggi Province, Daegu and Busan where it is already compulsory to wear a mask in public.

Of locally transmitted cases, the overwhelming majority were registered in the Greater Seoul area -- 138 in Seoul and 124 in Gyeonggi Province, which surrounds the capital. Incheon saw 32 new cases.

In a sign of the virus spreading rapidly beyond the greater Seoul area, however, cases were reported in other major cities and provinces – 15 each in Gangwon Province, Gwangju and Daejeon, 14 in South Jeolla Province, 10 in South Chungcheong Province, eight in South Gyeongsang Province, six in Daegu, three each in Ulsan, North Chungcheong Province, two each in Busan and North Gyeongsang Province.

Amid simultaneous outbreaks, health authorities are struggling to trace contacts, with transmission routes for 16.2 percent of the new cases reported for the past two weeks unidentified as of Sunday.

At the center of the sprawling outbreak is the Sarang Jeil Church and the massive rally held on Aug. 15 in central Seoul.

Anonymous COVID-19 testing will be provided until Aug. 26 at local health clinics to those who attended worship services at the church or the rally, the Seoul Metropolitan Government announced Sunday.

The authorities identified 45 more cases traced to the Seoul-based church as of Sunday noon, bringing the total number of the church-related cases to 841. A total of 112 COVID-19 cases of secondary transmission were confirmed from 21 establishments including other churches, nursing homes, hospitals and workplaces.

A total of 136 cases were reported in connection with the rally as of Sunday noon, up 32 from a day earlier. Five of the cases involve police officers who had been dispatched to the scene.

Cases tied to other clusters from churches, supermarkets to hospitals and franchise cafes are also rising.

The health authorities will discuss and review whether, when and how to raise the level of social distancing to the highest Level 3 while closely monitoring the pace, size and patterns of the COVID-19 outbreaks, the KCDC director said.

Despite calls from many experts for the government to put in place the toughest social distancing rules, authorities appear reluctant to do so in fear of the negative impact on the country’s economy and people’s everyday lives.

The government considers raising the social distancing level to the highest level when the number of daily virus cases reaches a two-week average of 100-200 and the doubling of new COVID-19 cases occurs more than twice a week.

Of the 10 imported cases, six were identified while the individuals were under mandatory self-quarantine in Korea, with the other four detected during the quarantine screening process at the border. Six of the newly diagnosed people were foreign nationals. Nine came from Asia and one from Europe.

So far, 14,200 people, or 81.61 percent, have been released from quarantine upon making full recoveries, up 31 from a day earlier. Some 2,890 people are receiving medical treatment under quarantine. Thirty people are in serious or critical condition.

The death toll remains unchanged at 309. The overall fatality rate amounted to 1.78 percent -- 2.08 percent for men and 1.53 percent for women -- as of Sunday. The fatality rate is 22.9 percent for those in their 80s or over and 7.72 percent for those in their 70s. 

The country has carried out 1,791,186 tests since Jan. 3, with 47,564 people awaiting results as of Sunday.

By Ock Hyun-ju (laeticia.ock@heraldcorp.com)