Three safety workers based at a central Seoul subway station have tested positive for the new coronavirus, the city's transportation operator said Wednesday, raising concern over a possibly bigger outbreak in the capital.

"The three who have tested positive were temporarily hired by a construction company, and their job was to prevent people from entering dangerous areas," Seoul Metro, which runs Seoul subways lines No. 1 to 8 and part of Line No. 9, said in a statement.

"All workers were found to have worn masks while working," it added.

Construction work was being carried out at the Line No. 2 section of City Hall station.

Of the three patients, one patient, who is a resident of Bucheon, west of Seoul, first tested positive on Monday. His infection route has not yet been identified.

The other two are residents of Anyang and Seongnam, both south of Seoul. Details of the other two patients were not immediately available.

All three are known to be elderly males.

The transportation operator has disinfected the subway station.

Of the 13 workers at the scene, three have tested negative, while six are waiting for their results. One has been placed under self-isolation as the person is not yet able to visit a screening center.

In addition, 39 people, including janitors and other employees at the station, are being checked for the virus as a precautionary measure.

City Hall Station, located near Seoul City Hall in Jung Ward, is one of the most centrally located subway stations on Lines No. 1 and 2. (Yonhap)