With at least 50 cases, a call center in southwestern Seoul emerged as the biggest COVID-19 infection cluster here Tuesday, triggering concerns over potential mass transmission in the metropolitan area that is home to 25 million, or nearly half of the country's population.

A high-rise in the neighborhood of Sindorim was closed and disinfected late Monday after workers at a call center on its 11th floor were confirmed as patients, according to Guro Ward, which oversees Sindorim.

In a meeting with other provincial officials, Seoul Mayor Park Won-soon said 64 cases occurred from the outbreak, while the Korea Centers for Disease Control and Prevention (KCDC) earlier said 50 cases have been traced to the call center.

A total of 46 employees who live in Seoul, Incheon and Gyeonggi Province have tested positive, in addition to four family members of them. The KCDC said all diagnosed employees worked on the 11th floor, without wearing masks.

But numbers are feared to further soar, considering the usually packed work environment at call centers, where temporary workers are outsourced to respond to customer complaints and inquiries.

Call center employees usually have to work in narrow booths and in the case of those working for financial firms, have to work in designated areas, as they handle personal information.

Health authorities said they plan to also check 550 other employees at the call center who worked on the seventh to ninth floors of the same building.

The infection is so far the biggest group infection that has occurred in the Seoul metropolitan area, raising worries of a widespread outbreak in the capital in addition to the mass infections in Daegu and North Gyeongsang Province in the southeast of the country.

"All 207 workers have been quarantined and inspections are taking place. This is the biggest infection reported in Seoul so far, and we are looking at this in a serious and grave manner and striving to prevent additional cases," Seoul Mayor Park Won-soon said in a press briefing.

The city government said it has launched an emergency taskforce of 30 researchers to look into the outbreak and has urged residents in the building, which has 19 floors and six basement levels, to isolate themselves.

In addition to the offices and commercial facilities in floors one to 12, 140 residential flats are located on the top seven floors of the building in Sindorim, one of the busiest areas in southwestern Seoul, where subway lines No. 1 and 2 pass through.

An inspection center has been set up on the ground floor of the building for residents and employees to be screened for the new coronavirus, the city said.

The latest cluster of infections was identified at the call center operated by Metanet Mplatform, whose clients include the local affiliate of global insurance group Chubb.

The outbreak, the first to occur in a call center, raised worries about similar office environments in which employees have to work in close proximity to each other and cannot afford to take part in the government's social distancing campaign.

The Seoul mayor said the city plans to monitor similar work environments and advise owners of businesses, such as karaoke rooms or dance clubs, to suspend operations for the time being.

Around 80 percent of the more than 7,000 infections reported in South Korea are attributed to group transmissions, according to KCDC data.

While the minor religious sect of Shincheonji is associated with more than 60 percent of all infections, infections have also occurred in minor clusters in hospitals, religious services and even Zumba classes.

In Seoul, 14 infections occurred at the Eunpyeong St. Mary's Hospital, a general hospital in northwestern Seoul, and 13 in a residential complex in the Seongdong Ward as of Monday, according to the KCDC.

South Korea has reported 7,513 infections since its first confirmed case on Jan. 20. (Yonhap)