More than 80 foreigners have tested positive for COVID-19 in Dongducheon, north of Seoul, officials said Tuesday.

The local government has been testing all foreign nationals living in the city 40 kilometers north of the capital after neighboring cities, such as Yangju, reported increases in the number of foreigners infected with COVID-19.

A total of 509 people got tested on Sunday, including foreign residents and locals, according to the authorities. The city currently has 3,966 foreign residents.

The results came back positive for five foreign nationals on Monday and 79 foreigners on Tuesday, bringing the total to 84, the local government said.

Four South Koreans also tested positive over the two days.

No clues about whether or how the cases were interconnected have been found yet, local authorities said.

"Other than the fact that there's been a mass infection among foreign nationals, there's nothing particular we can confirm," a local government official said. "We haven't even figured out what types of jobs these foreigners have."

The number of COVID-19 infections in the city could rise further as mass testing of all foreign residents is still under way in the city.

Among those already confirmed to be infected with the virus, the city will also run tests for more contagious COVID-19 variants, according to officials. 

On Feb. 18, the local government sent notices to churches and other communities with large foreign populations to urge them to get tested for COVID-19.

Health authorities have begun an epidemiological survey to trace the infection routes in the latest cases.

The new school year began on Tuesday, but all schools in Dongducheon switched to remote learning on Tuesday and Wednesday. (Yonhap)