South Korean health authorities were put on edge again on Tuesday as yet another cluster case looms, at a major hospital in Seoul, following mass infections tied to nightclubs and bars in the nightlife district of Itaewon.

Four nurses working at Samsung Medical Center, one of the country's five major general hospitals, were confirmed to have been infected with COVID-19, and more related cases are likely to come as transmission routes are unknown, according to the Korea Centers for Disease Control and Prevention (KCDC).

A 29-year-old nurse working at an operating room of the hospital tested positive for COVID-19 Monday and three others were confirmed to have been infected earlier in the day, according to health authorities and the Seoul city government.

The 13 virus cases detected Monday brought the total caseload to 11,078, according to the KCDC. Of them, nine were local infections, seven of which were tied to the Itaewon outbreak.  

The outbreak at the general hospital put health authorities back on high alert after respite from club-linked infections in the district of Itaewon in Seoul.

KCDC Deputy Director Kwon Joon-wook said 277 people, including 262 health workers, came into contact with the infected nurses.

The first infected nurse neither visited Itaewon clubs nor came into contact with visitors to the neighborhood.

"The number of virus patients will likely increase, given its 14-day incubation period. There is a high possibility that infections of health workers will rise," Kwon said at a briefing.

Kwon said an epidemiological study is under way to determine the source of COVID-19 infections at the hospital.

Samsung Medical Center has shut down 25 operating rooms, including a surgery room where the infected nurses worked. It will not accept new inpatients for the next three days.

A Seoul city official said the infected nurses took part in a surgery in a negative pressure operating room last week.

Samsung Medical Center is on high alert over the COVID-19 pandemic, as 85 out of the country's 186 Middle East Respiratory Syndrome (MERS) cases in 2015 were reported from the hospital.

The KCDC said a total of 266 medical workers here have been infected with the coronavirus so far.

South Korea has successfully flattened the virus curve with massive testing and contact tracing, with its daily infections falling to a single digit figure in the first week of May.

But Itaewon emerged as a hotbed for cluster infections, after a 29-year-old man tested positive on May 6 with visits to five clubs in the district in early May. The total caseload tied to the Itaewon outbreak reached 187 as of Tuesday noon, with additional 12 patients linked to the cluster being reported over past 36 hours.  

"We urge citizens to implement quarantine steps in their daily routines as if we lower our guard against the virus, new infections can occur at any time," Yoon Tae-ho, a senior health official, said in a separate briefing.

South Korea's daily infections of the new coronavirus stayed below 20 for the fourth straight day Tuesday due to a marked slowdown in cluster infections liked to Itaewon.

The country added four more imported cases. It reported no additional death, bringing the death toll to 263. The fatality rate was 2.38 percent.

The total number of people released from quarantine after full recoveries stood at 9,938, up 34 from the previous day.

The country, with a population of more than 50 million, has carried out 765,574 tests since Jan. 3, including 12,363 the previous day.

As club-linked cases slowed, the country will reopen schools Wednesday starting from high school seniors as scheduled.

South Korea switched an "everyday life quarantine" scheme on May 6 to enable citizens to carry out social and economic activities under quarantine rules.

The KCDC said it will draw up specific quarantine measures that can be applied to highly crowded facilities, based on the risk of virus spread. Health authorities said they are considering mandatory imposition of core quarantine guidelines on such facilities. (Yonhap)