Four nurses working at Samsung Medical Center have tested positive for the novel coronavirus, authorities said Tuesday, raising concerns over the first infection involving health workers at a major hospital here.

Four nurses working at the cardiothoracic and ob-gyn operating rooms in the hospital in southern Seoul tested positive on Monday and Tuesday, local government officials said Tuesday.

Of the 277 patients and hospital workers who must be checked for the virus, 265 have undergone screening. Around 100 of them are waiting for the results, while the remaining 12 will be checked later in the day, according to the office of Gangnam Ward, where the hospital is located.

As a precautionary measure, the hospital has closed down 25 operating rooms on the third floor of the main building and will not treat new in-patients for the next three days, the ward office said.

The case, the first time in which medical workers at a major general hospital have been infected, has put authorities on alert over the possibility of a bigger outbreak.

Authorities have yet to identify the infection route, heightening concerns over the source of the transmission.

"An epidemiological investigation is under way to determine whether the infection occurred within the hospital, and if so, what the source of infection is, or whether it came from outside the hospital," said Kwon Joon-wook, deputy director at the Korea Centers for Disease Control and Prevention (KCDC).

The new group infection began when a nurse working at the hospital's cardiothoracic operating rooms tested positive Monday. Three other nurses later were confirmed to be infected.

The first nurse did not report for work during the weekend and showed symptoms of a fever Sunday night, according to the health authorities.

The hospital said the nurse had not visited Seoul's popular nightlife district of Itaewon, which has recently become an infection cluster.

The country added 13 new COVID-19 cases Tuesday, bringing the total caseload to 11,078. Of the number, nine were local infections.

The total caseload in the capital has reached 742, up six from the previous day.

Samsung Medical Center is one of the country's biggest general hospitals, with around 8,900 employees and 2,000 beds. Up to 9,700 outpatients are estimated to visit the hospital daily.

The hospital, which had reported scores of patients during the 2015 Middle East respiratory syndrome (MERS) outbreak, has been checking all employees for virus symptoms twice a day and all visitors at its entrance. It has also limited the number of visitors per patient to one. (Yonhap)