South Korea reported an eight-fold jump in the number of coronavirus cases on Saturday.
Officials in the country say two people have died from the viral pneumonia, with more than 400 cases of the illness, officially called Covid-19.
Though initial infections in South Korea were linked to China, new ones have not involved international travel - raising concerns over the ease with which the illness can spread.
In response, hundreds of schools have been shut, churches have told worshippers to stay away and some mass gatherings have been banned.
On Thursday officials in one major city in South Korea urged its 2.5 million residents to avoid going outside as cases of the virus spiked.
The World Health Organisation (WHO) warned that such clusters not directly linked to travel from China 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."
Mr Tedros pointed to Iran’s discovery of 18 cases and four deaths in two days. On Saturday Iranian state media reported a sixth fatality from the new virus
Globally, more than 76,000 people have been infected in 27 countries, and more than 2,200 have died.
"These dots are very concerning – take them as dots or trends," Mr Tedros said.
South Korea Prime Minister Chung Se-kyun opened a government meeting on the health emergency by saying: "We have entered an emergency phase.
"Our efforts until now had been focused on blocking the illness from entering the country.
"But we will now shift the focus on preventing the illness from spreading further in local communities."
Daegu, a south-eastern city of 2.5 million people that is the country’s fourth largest, emerged as the focus of government efforts to contain the disease.
A total of 110 infections have been confirmed in the city and surrounding areas, including South Korea’s first fatality from the virus.
Most of the cases have been linked to a single house of worship, a branch of the Shincheonji Church of Jesus.
South Korea saw 20 new cases of the virus reported on Wednesday, 53 on Thursday and 100 on Friday.
The first three cases in the country’s 600,000-member military also sprang up on separate bases on Friday, adding to concern about the spread.
A sailor on Jeju Island, an army officer in North Chungcheong province and an air force officer on temporary assignment at the military headquarters in central South Korea all tested positive for coronavirus.
Officials said all three had links to Daegu.
In China, the epicentre of the virus and where the vast majority of cases have occurred, officials have expressed optimism over the number of new infections - which has been trending downward.
China said on Friday 889 new cases were recorded in the preceding 24 hours and 118 additional deaths.