Twenty-one Vietnamese students living in a college dormitory in a central South Korean city have been diagnosed with COVID-19, local officials said Wednesday, reporting a rare coronavirus cluster of foreign nationals here.

The nation's latest mass infection occurred at Ajou Motor College in Boryeong, a South Chungcheong Province city about 200 kilometers south of Seoul.

The college reported its first confirmed case Monday, when a Vietnamese student residing in its dormitory tested positive for the coronavirus.

Municipal health authorities then conducted coronavirus tests on everyone suspected of having come into contact with the first patient -- 67 foreign dormitory residents (50 from Vietnam, 16 from Uzbekistan and one from Thailand), 64 Korean dormitory residents and 31 school personnel.

Through the tests, 20 Vietnamese students have been confirmed to be infected, the officials said, noting authorities are to administer coronavirus tests to all 400 other dormitory residents and school officials Wednesday.

The municipal authorities are investigating the patients' travel and infection routes, suspecting that the group infection may have occurred due to close contact in enclosed dormitory rooms.

According to some municipal officials, the infected Vietnamese students cooked and ate food together in a separate kitchen area within the dorm.

Notably, 13 of the infected Vietnamese students reportedly worked part-time at a restaurant on Daecheon Beach in the same city on a recent weekend, prompting authorities to disinfect the establishment and conduct coronavirus tests for its employees.

Boryeong, a city of slightly over 100,000 people that had reported only 34 confirmed cases prior to the college dorm cluster, raised its social distancing level to 2.5, the second highest in the nation's five-tier system, from the current 2, at noon Wednesday.

"The present situation is very grave. All citizens are required to cancel all year-end meetings and events, and thoroughly comply with quarantine and social distancing rules," Boryeong Mayor Kim Dong-il said.

In recent weeks, a string of clusters of coronavirus infections have erupted at elderly nursing homes, churches, educational institutions and other multiuse facilities throughout South Korea, as the nation has reported around 1,000 new cases every day. (Yonhap)