South Korea on Saturday reported an eight-fold jump in viral infections in four days to 433, most of them linked to a church and a hospital in and around the nation's fourth-largest city, where health workers scrambled to screen thousands of worshippers. Later Saturday, the U.S. State Department issued two travel advisories warning Americans to "exercise increased caution" when traveling to both South Korea and Japan.

The country also reported its third death from the virus, a man in his 40s who was found dead at home and posthumously tested positive.

Vice Health Minister Kim Gang-lip said the outbreak had entered a serious new phase, but still expressed cautious optimism that it can be contained to the region surrounding Daegu, where the first case was reported on Tuesday.

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Of the 229 new cases in South Korea, 200 are from Daegu and nearby areas. By Saturday morning, the city of 2.5 million and surrounding areas counted 352 cases, including two fatalities in the Cheongdo hospital. Both patients had pneumonia.

"Although we are beginning to see some more cases nationwide, infections are still sporadic outside of the special management zone of Daegu and North Gyeongsang Province," Kim said during a briefing. He called for maintaining strong border controls to prevent infections from China and elsewhere from entering South Korea.

South Koreans wearing masks to prevent the coronavirus ride on a bus on February 22, 2020 in Seoul. Getty Images

Nationwide, the numbers told of a ballooning problem. There were 20 new cases reported Wednesday, 53 on Thursday and 100 on Friday.

Around 230 of those have been directly linked to a single house of worship, a Daegu branch of the Shincheonji Church of Jesus, where a woman in her 60s attended two services before testing positive for the virus.

Officials are also investigating a possible link between churchgoers and the spike in infections at the Cheongdo hospital, where more than 110 people have been infected so far, mostly patients at a mental illness ward.

Health officials were screening some 9,300 church followers, and said that 1,261 of them have exhibited cough and other symptoms.

Among them, four had traveled abroad in recent months, including one to China, although that trip came in early January and was not near Hubei.

All 74 sites operated by the Shincheonji Church have been closed and worshippers have been told to instead watch services online for a sect whose leader claims to be an angel of Christ, but who is dismissed by many outsiders as a cult leader. Its teachings revolve largely around the Book of Revelation, a chapter of the New Testament known mostly for its apocalyptic foreshadowing.

Health and city officials say the woman who first tested positive had contact with some 1,160 people, both at the church, a restaurant and a hospital where she was treated for injuries from a car accident.

But officials say it's unlikely that the woman set off the chain of infections, and that she was probably just the first person to be detected in an area where the virus was circulating in the population.

In some positive news, China said Saturday that the daily count of new virus cases there fell significantly to 397, though another 109 people died of COVID-19, the disease caused by the virus. The new figures, along with an upward revision of Hubei's earlier count, brought the total number of cases in mainland China to 76,288, with 2,345 deaths.

China has severely restricted travel and imposed strict quarantine measures to stop the virus from spreading. But a few Chinese provinces, eager to restart factories and their economies, began easing those restrictions after reporting no new cases in recent days. Liaoning and Gansu provinces both lowered their emergency response level, and two cities in Shaanxi province resumed bus services and removed some checkpoints.