South Korea says the number of new coronavirus cases in the country has more than doubled in one day.
Officials say 229 new cases have been confirmed since Saturday, raising the total to 433.
In a televised address, Prime Minister Chung Sye-kyun described the situation as "grave".
Many of the new cases are linked to a hospital and to a religious group near the south-eastern city of Daegu, authorities have said.
Two patients in South Korea have died so far and there are fears the number will rise.
Daegu and nearby Cheongdo - where the hospital is situated - have been declared "special care zones" and the streets of Daegu are reported to be largely abandoned.
South Korea has now reported the largest number of confirmed infections after China - 76,288 cases including 2345 deaths - and the outbreak on the Diamond Princess cruise ship off the coast of Japan which has seen more than 600 cases.
The latest developments came as Chinese health authorities reported a decrease in the rate of deaths and new cases of the coronavirus.
The head of the World Health Organization (WHO), Dr Tedros Adhanom Ghebreyesus, has expressed concern at the number of new cases with no clear link to China or other confirmed cases.
He said the greatest concern now was countries with weaker health systems, particularly in Africa.
Outside China, more than 1200 cases of the virus have been confirmed in 26 countries and there have been at least eight deaths, the WHO says.
An evacuation flight carrying 32 British and other European cruise ship passengers from Japan has landed in England
Italy reported its second death from the virus - a woman living in the northern region of Lombardy - a day after a 78-year-old man became the first person in Italy to die from the new coronavirus.
Iran reported its fifth death from the disease. The outbreak is centred on the holy city of Qom but officials warned that the virus may already have spread to "all cities in Iran".
The new virus, which originated last year in Hubei province in China, causes a respiratory disease called Covid-19.
Medical officials first announced 142 new cases and then hours later increased the number by 87.
In a statement, the Korea Centers for Disease Control and Prevention (KCDC) said that of the 229 new cases, 95 were related to Daenam Hospital in Cheongdo, which treats the elderly and people with mental health issues. There are now 114 confirmed cases at the hospital - nine staff and 102 patients - it added.
Vice Health Minister Kim Gang-lip said the outbreak had entered "a serious new phase".
He said most of those diagnosed at the hospital had been inpatients for mental illnesses. Some cases showed "no confirmed epidemiological connections", meaning it was not clear how they had contracted the virus, he added.
A 63-year-old man died at the hospital on Wednesday and another patient died on Friday in Busan after being transferred from Daenam Hospital, Yonhap news agency reports.
In his national address, Prime Minister Chung asked South Koreans to avoid large gatherings, including religious events.
"The government believes the situation of the Covid-19 infections is turning into a grave phase, and is making every effort to prevent the spread," he said.
Mr Chung said the government would take action against anyone hoarding face masks or taking part in now-banned mass rallies.
KCDC said 62 new cases - 231 in total - were connected to a Christian sect in Daegu called the Shincheonji Church of Jesus.
Authorities have highlighted that a large number of sect followers attended the funeral of the founder's brother from 31 January to 2 February.
The agency said it had placed 9,336 Shincheonji members in self-quarantine and that, of those, more than 500 were being tested for the coronavirus.
In a briefing to the African Union, Dr Tedros said the WHO was working with African governments to train thousands of health workers and to provide equipment to identify and treat infected people.
"Our biggest concern continues to be the potential for Covid-19 to spread in countries with weaker health systems," he said. "We are working hard to prepare countries in Africa for the potential arrival of the virus."
Dr Tedros said some countries, such as the Democratic Republic of Congo, were using experience gained from testing for the Ebola virus to test for the new coronavirus.
"This is a great example of how investing in health systems can pay dividends for health security," he said.
A team of experts from the WHO is due to arrive in the Chinese city of Wuhan today, where the outbreak first emerged.