SEOUL, South Korea — South Korea on Saturday reported a six-fold jump in viral infections in four days to 346, most of them linked to a church and a hospital in and around the fourth-largest city where schools were closed and worshipers and others told to avoid mass gatherings.

Initial infections were linked to China, but new cases in South Korea and Iran — where there have been four deaths — don’t show a clear connection to travel there. In an emerging cluster of illnesses in northern Italy, the first to fall ill met with someone who had returned from China on Jan. 21 without experiencing any symptoms of the new virus, health authorities said.

China said Saturday the daily count of new virus cases there fell significantly to 397, with another 109 people dying of the disease, most in the epicenter of Hubei province.

The new figures bring the total number of cases in mainland China to 76,288 with 2,345 deaths, as strict quarantine measures and travel bans continue to contain the disease that emerged in China in December and has since spread world-wide. The daily figure is down from 889 on Friday.

Of the 142 new cases in South Korea, 131 are from Daegu and nearby regions, which have emerged as the latest front in the widening global fight against COVID-19.

The World Health Organization warned that clusters not directly linked to travel, such as the ones in South Korea and Iran, suggest that time may be running out to contain the outbreak.

“The window of opportunity is still there. But our window of opportunity is narrowing,” said WHO Director-General Tedros Adhanom Ghebreyesus. “We need to act quickly before it closes completely.”

Tedros singled out Iran’s discovery of 18 cases and four deaths in two days — and that a traveler from Iran carried the virus to Lebanon, and another traveler from Iran to Canada.

“These dots are very concerning — take them as dots or trends,” he said.

South Korea Prime Minister Chung Se-kyun started a government meeting on the health emergency Friday by saying, “We have entered an emergency phase.”