A total of 297 COVID-19 cases had been traced to six churches and mission schools run by a Christian organisation, senior health official Yoon Tae-ho told a briefing on Wednesday.

More than 100 cases were confirmed overnight among people linked to a church and its mission school in Gwangju, south of Seoul, officials said. Another 171 cases had been linked to an affiliated school in the city of Daejeon since January 17.

The Korea Disease Control and Prevention Agency said the Daejeon mission school outbreak appeared to have been spreading for some time before it was detected.

The Christian organisation responsible for the facilities, International Mission, was ordered to test everyone linked to 32 of its 40 schools and churches around the country.

The group apologised for not taking early measures to prevent the outbreak. It said that while some infected students may have been asymptomatic it had also failed to require students with cold-like symptoms to get tested.

Prime Minister Chung Sye-kyun called on people affiliated with the affected facilities to get tested.

"The key is speed. I call on the authorities and local governments to make all-out efforts to identify related facilities and prevent further transmission," Chung told a government briefing.

The KDCA reported 559 new cases as of midnight on Tuesday, up from 354 a day before, bringing the national tally to 76,429 infections with 1,378 deaths.

South Korea had managed to keep the virus under control thanks to aggressive testing and contact-tracing but a third wave that broke out late last year has proven more difficult to contain.

Australian Associated Press