Thousands of members of a secretive religious sect in South Korea are being screened for the new coronavirus after more than 430 cases were confirmed in the country by officials, one of several fresh clusters of the disease globally.

More than 78,000 people around the world have been infected by the Covid-19 virus, with most cases in mainland China, though clusters that have unclear origins have emerged in Singapore, Iran and South Korea.

In Iran, a sixth death was confirmed on Saturday, prompting Tedros Adhanom Ghebreyesus, the head of the World Health Organization, to say he was especially concerned about the country. So far, 28 cases have been identified across four cities, but experts have speculated that many more infections are likely to remain undetected, and it is believed cases that have since appeared in Canada and Lebanon are linked to Iran. Infections in the country are believed to have begun in the city of Qom, but it is unclear how.

There is also uncertainty in Singapore, where eight out of 85 infections appear to have no links to previous cases, and in South Korea, where links to a controversial church are being investigated.

Iranians wearing face masks in Tehran. Photograph: Abedin Taherkenareh/EPA

On Saturday, Ghebreyesus expressed concern about cases that have been identified where a patient has had no contact with a confirmed infected person, and has not travelled to China.

“Although the total number of Covid-19 cases outside China remains relatively small, we are concerned about the number of cases with no clear epidemiological link,” he said.

Most people who contract the disease experience mild symptoms, making transmission hard to trace. The virus’s fatality rate is about 2%, according to initial data from China.

Andrew Tatem, a professor within geography and environmental science at the University of Southampton, said: “Prior to the recent news of apparent outbreaks in South Korea, Italy and Iran, it seemed like the relatively low number of cases outside of China and with no link to travelling there pointed to hope that the virus could be contained. This recent news, particularly regarding Iran, is especially worrying.”

In the South Korean city of Daegu, where most of the country’s cases have been concentrated, officials are racing to screen worshippers who they believe may have been exposed to the virus at a service run by a controversial church. The city’s 2.5 million people have been advised to stay in their homes if possible, while officials are sending medical supplies and clinical staff. Across Daegu and nearby areas, 354 cases have been recorded.

Q&A How can I protect myself and others from the coronavirus outbreak? Show The World Health Organization is recommending that people take simple precautions to reduce exposure to and transmission of the coronavirus, for which there is no specific cure or vaccine. The UN agency advises people to: Frequently wash their hands with an alcohol-based hand rub or warm water and soap

Cover their mouth and nose with a flexed elbow or tissue when sneezing or coughing

Avoid close contact with anyone who has a fever or cough

Seek early medical help if they have a fever, cough and difficulty breathing, and share their travel history with healthcare providers

Advice about face masks varies. Wearing them while out and about may offer some protection against both spreading and catching the virus via coughs and sneezes, but it is not a cast-iron guarantee of protection Many countries are now enforcing or recommending curfews or lockdowns. Check with your local authorities for up-to-date information about the situation in your area. In the UK, NHS advice is that anyone with symptoms should stay at home for at least 7 days. If you live with other people, they should stay at home for at least 14 days, to avoid spreading the infection outside the home.

Yoon-sook Lee, a restaurant owner in Yulha-dong, Daegu, said the city’s streets were deserted. “Those who are outside are half-running as if they are all in a hurry or being chased,” she said, adding that people had been stockpiling water and instant noodles.

“I’m starting to feel panicky as I’m worried about my family. I have young children so I’m worried about them. I’m also getting really worried about how long this will go on as it will affect our family’s income,” she said. Her restaurant provides catering for events, but all orders have been cancelled.

About 230 cases in South Korea have been linked to the Daegu branch of the Shincheonji Church of Jesus, while scores of cases on a mental health ward at Cheongdo hospital may also be related to the sect. The exact source of these infections remains unclear. While health officials are trying to trace more than 1,000 people who had contact with a 61-year-old woman who attended services at the church, it is believed the virus was present before she tested positive.

A South Korean health official sprays disinfectant in front of a hospital in Cheongdo county near Daegu. Photograph: Yonhap/AFP via Getty Images

The church said in a statement on its website that it had more than 120 students in China enrolled on its bible course but had no physical building or meeting place.

“The church leaders have told us that this is a test by the devil as he is jealous of the expansion of our church and I believe that is the case,” said one of the church’s members, who asked not to be named but said she had not attended the service linked to the outbreak. “I hope that this situation will make our church stronger. We get persecuted by other people as it is, so this situation is even tougher for us.”

The church is accused by many of being a cult and its members often remain distant from outsiders and do not disclose their beliefs to family and friends. Its leadership has shared the names of people who attended its services with the authorities, and about 1,000 are being assigned a government worker to monitor for any symptoms. Secrecy surrounding the sect, however, means such contact-tracing work is even more challenging.

Worshippers attend small local services twice a week, as well as larger gatherings – such as the Daegu service linked to the current cluster of cases. These bigger services can attract hundreds of people from across the country, where the church has a total of 230,000 members. Attendees kneel closely together on the floor, chant, and shake hands with those around them.

The church said it had shut all branches and centres and was disinfecting its facilities. Members sat on the floor at services in order to allow as many people to attend as possible, it added.

Across the city, large gatherings have been banned and nurseries closed, while soldiers have been forbidden from leaving their barracks after several military personnel tested positive. A Samsung plant in Gumi, near Daegu, that employs hundreds of people has also been closed after a worker tested positive.

Italy has recorded two deaths linked to the virus, prompting officials to place dozens of towns in Lombardy and Veneto, in the north of the country, under effective lockdown. Confirmed case numbers in the country have risen to 79.

In China, where more than 2,300 people have died in the outbreak, there were 397 additional cases on Friday.

Tedros said the biggest concern continued to be the potential for Covid-19 to spread in countries with weaker health systems.

Associated Press contributed to this report.