South Korea's new virus cases rose by the highest number in nine days Wednesday amid looming mass infections at a major hospital in Seoul and a rise in nightclub-linked cases.

An uptick in virus cases put health authorities on higher alert over further community spread as high school seniors began to return to schools following a monthslong school closure over the pandemic.

The country added 32 new coronavirus cases, raising the total caseload to 11,110, according to the Korea Centers for Disease Control and Prevention (KCDC). Of the new COVID-19 cases detected Tuesday, 24 were local infections.

The daily new infections mark the largest since May 11, when the number hit 35.

Concerns about yet another cluster of infections emerged, as four nurses working at Samsung Medical Center, one of the country's five major general hospitals, were infected with COVID-19.

Health authorities said more related cases are likely to spring up, as transmission routes are unknown.

Contact tracing is being conducted to determine whether the source of infections originated in the hospital. Health authorities have conducted virus tests on 1,199 people, 641 of whom tested negative.

The KCDC said the nurses may have been infected at shared places inside the hospital like fitting rooms or through patients undergoing surgery.

"We've yet to detect a link between their infections and club-related cases in Itaewon," KCDC Director Jeong Eun-kyeong said in a briefing.

Jeong said health authorities need to continue monitoring for at least one week to assess whether the nurse cases could develop into mass infections at the hospital, given a two-week incubation period.

Cases tied to clubs in Seoul's nightlife district of Itaewon have not evolved into explosive infections but continued to increase steadily. The total caseload linked to the Itaewon cluster reached 196 as of noon Wednesday, with nine related patients reported over past 36 hours. Of them, 100 cases were reported in Seoul.

Health authorities remain vigilant against further community spread at a time when more schools are set to reopen in the coming weeks.

The education ministry went ahead with phased school reopening as scheduled amid a letup in Itaewon-linked infections.

But as precautionary measures, Incheon, west of Seoul, and Anseong, south of the capital, had to order third graders at 75 high schools to go home immediately.

Two high school students tested positive in Incheon, in what could be infections tied to the Itaewon outbreak.

In Gyeonggi Province, which surrounds Seoul, 27 students were moved to coronavirus testing sites as they showed virus symptoms, like fever.

Health authorities said COVID-19 infections appear to be well contained in risk-prone facilities, such as churches and hospitals, where a bulk of cases were reported at an initial stage of the virus outbreak here.

"But virus infections are spreading at facilities like clubs and karaoke," Vice Health Minister Kim Ganglip said in a briefing.

Despite a steady rise in cluster infections, Kim said it is not appropriate to immediately return to strict social distancing.

"Efforts to minimize virus infections should be prioritized through thorough quarantine measures in communities. Even if sporadic infections occur, it is not appropriate to return to the stricter social distancing system," Kim added.

Following 45 days of stricter social distancing, South Korea switched to an "everyday life quarantine" scheme on May 6 to enable citizens to carry out social and economic activities under quarantine rules.

The country, meanwhile, added eight imported cases and reported no additional deaths, with the total death toll staying at 263. The fatality rate was 2.37 percent.

The total number of people released from quarantine after full recoveries stood at 10,066, up 128 from the previous day.

The country, with a population of more than 50 million, has carried out 776,433 tests since Jan. 3, including 10,859 the previous day. (Yonhap)