SEOUL, Jan 27 (Reuters) - South Korean authorities were scrambling on Wednesday to rein in coronavirus outbreaks centred on Christian schools as the country reported a jump in infections, dampening hopes of a speedy exit from a third wave of the pandemic.

At least 323 COVID-19 cases had been traced to churches and mission schools run by a Christian organisation in two cities, according to the Korea Disease Control and Prevention Agency (KDCA) data.

More than 100 cases were confirmed overnight among people linked to churches and its mission schools in Gwangju, about 270 kms (168 miles) south of Seoul, officials said. Another 176 cases had been linked to an affiliated school in the city of Daejeon since Jan. 17.

The health authorities 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.

'We deeply apologise for not responding earlier and for thinking the students could have had a cold when a student first developed fever,' it said in a statement.

The organisation has submitted a list of 841 students and staff in 11 facilities, Yonhap news agency reported, citing the health authorities.

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.

The KDCA has said 45.4% of infections in the country over the past year were caused by cluster infections emerging from specific close-knit groups. Religious facilities were the main source of such clusters.