HONG KONG — An alarming surge of new coronavirus cases outside China, with fears of a major outbreak in Iran, is threatening to transform the contagion into a global pandemic, as countries around the Middle East scrambled to close their borders and continents so far largely spared reported big upticks in the illness.
In Iran, which had insisted as recently as Tuesday that it had no cases, the virus may have reached most major cities, including Tehran, and has killed at least four people, according to health officials. Already, cases of travelers from Iran testing positive for the virus have turned up in Canada and Lebanon.
The number of cases also soared in South Korea, with the sudden spread tied to a secretive church where hundreds of congregants attended services with numerous people infected with the virus.
The United States now has 34 cases, with more expected, and Italy experienced a spike from three cases to 17 and ordered mandatory quarantine measures.
“The cases that we see in the rest of the world, although the numbers are small, but not linked to Wuhan or China, it’s very worrisome,” Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, said Friday at a news conference at the agency’s headquarters in Geneva. “These dots are actually very concerning.”
As uneasiness about the breadth and duration of the outbreak grew, stocks fell for the second straight day on Friday amid worries the virus would drag down global demand and hurt the world economy.
The disturbing reports out of Tehran suggested the virus was being transmitted far more widely there than officials had previously acknowledged. While the country’s health officials confirmed only 18 cases by Friday, the number of deaths indicates the total is likely to be far higher.
Four reported deaths probably mean at least 200 cases, said Michael T. Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. If the virus kills about 2 percent of known victims, as Chinese doctors have reported, then the number of deaths can be multiplied by 50 to get a rough case estimate, he explained.
“People don’t die right away of this virus — it usually takes two or three weeks after cases start to spread for the first death,” Mr. Osterholm said. “So there may be a lot more cases, and a lot more deaths on the way. And we didn’t even know there was a problem in Iran before yesterday.”
Minou Mohrez, who is on the infectious disease committee of the Iranian Health Ministry, told the official IRNA news agency on Friday that it was clear the virus was spreading across Iran’s cities.
“A coronavirus epidemic has started in the country,” she said. “It’s possible that it exists in all cities in Iran.”
A spokesman for the Health Ministry, Kianush Jahanpur, said on Friday there were more than 735 people hospitalized with flulike symptoms who were being tested for the virus.
Kuwait’s civil aviation authority on Friday stopped all flights to and from Iran, which shares a long border with both Afghanistan and Iraq, where health officials have a limited capacity to stop the spread of the virus should it find its way to those countries.
Dr. Sylvie Briand, the director of infectious hazards management for the W.H.O., said the rapid increase in cases in Iran was disquieting.
“We are wondering what the extent of the outbreak in Iran is,” she told reporters on Friday. “We are wondering about the potential for more cases to be exported in the coming days. We want all countries to be aware of this and to put in place detailed measures to pick up these cases as early as possible.”
As concern grew that Iran was emerging as an important new vector of transmission, the country where the coronavirus originated was also responding to significant negative developments.
Officials in China, already straining to deal with an outbreak that has infected more than 76,000 people and resulted in 2,300 deaths, announced a new front in its war on the virus on Friday as officials reported clusters of infections in at least four prisons in three provinces.
The outbreaks, affecting at least 512 prisoners and guards, raised the specter of the disease spreading through the country’s extensive prison system.
More than 200 of the infections occurred in one prison in the city of Jining, 450 miles east of Wuhan, the capital of Hubei Province and the center of the outbreak; officials there suggested that the cluster may have been tied to a prison guard.
In South Korea, the total number of cases surpassed 200 on Friday, and the authorities were racing to trace all the people who have come in contact with members of the Shincheonji Church of Jesus, whose members account for two-thirds of the new infections in the country.
More than 540 other church members have reported potential symptoms, health officials said, raising the possibility that the nation’s caseload could skyrocket.
As of Friday, more than 340 members of Shincheonji, which mainstream South Korean churches consider a cult, still could not be reached, according to health officials, who were hoping to screen them for signs of infection.
In response, the government is shutting thousands of kindergartens and community centers, even banning the outdoor political rallies that are a feature of life in downtown Seoul.
All four virus-related deaths in Iran occurred in Qom, a holy city popular with Shiite pilgrims across the Middle East.
People have already tested positive in Qom, Tehran and Gilan, near the Caspian Sea, said Mr. Jahanpur, the Health Ministry spokesman.
“Most of these people were residents of Qom or they had traveled to Qom in the past days or weeks,” he said.
In Qom, schools and religious seminaries were shut down on Thursday as officials urged people to avoid gathering in large groups. But on Friday, as Iranians went to vote in parliamentary elections, polling stations were open and the communal pools of ink for people to dip their fingers proving they voted were in wide use.
With rumors spreading across the country on instant messaging services like Telegram, a confused and increasingly worried public watched as Tehran’s largest metro station was suddenly closed. Workers wearing protective gear descended on the station, apparently responding to reports of sick commuters. It remained closed Friday night.
There was growing skepticism over the government’s handling of the outbreak. Mahmoud Sadeghi, an outspoken member of Parliament from Tehran, accused the government of “covering up the spread of an epidemic.”
While the source of the outbreak in Iran could not be pinned down, officials speculated that it began in the large population of Chinese workers in the country.
Critics accused the government of playing down the disease, and failing to take strict precautions to prevent its arrival in the country, in order to avoid provoking China, a key trading partner and a lifeline for Iran’s economy in the face of U.S. sanctions.
The sanctions against Iran could hamper its ability to contain the spread of the virus and diminish the country’s ability to mobilize international support.
“Iran does have problems accessing specialized medication for rare and special diseases because of sanctions — either private companies or banks refuse to work with Iran in fear of U.S. secondary sanctions,” said Tara Sepehri Far, an Iran researcher at Human Rights Watch.
The new global clusters showed, again, the difficulty in judging the true number of infections, amid concerns about underreporting and rapidly shifting definitions of confirmed cases.
Further bolstering the idea that the virus is spreading widely, an epidemiological modeling team from Imperial College in London estimated Friday that two-thirds of the people infected with coronavirus who left mainland China before restrictions were imposed had traveled throughout the world without being detected.
The team, one of several modeling groups regularly consulted by the W.H.O., calculated how many cases were detected in different countries and how many should have been detected based on flights that left Wuhan just before most air travel out of China ended.
Detection failures “potentially resulted in multiple chains of as-yet-undetected human-to-human transmission,” the modeling team’s study concluded.
The virus is spreading even in places that might be expected to have the closest monitoring and prevention. In Beijing, a spike in cases at two hospitals raised fears that the epidemic could be growing in a city so far largely exempt from extensive infections.
The infections — and in some cases, deaths — of medical workers have become a potent symbol of the epidemic’s toll for many Chinese. On Thursday, another doctor in Wuhan died. The doctor, Peng Yinhua, 29, had postponed his wedding to continue treating patients, according to a statement from the hospital where he worked.
Earlier this week, a high-profile doctor, Liu Zhiming, the director of the Wuchang Hospital in Wuhan, died.
The almost random nature of new reports and new deaths is an indication the virus is moving much faster than countries are reporting to the W.H.O., Dr. Osterholm said.
“How many of these clusters and travel cases and prison outbreaks do we have to see before we realize that we’re just seeing the tip of the iceberg?” he said. “Testing is just getting set up around the world. There’s barely any in Africa right now. Even in the U.S., we’re testing travel cases — but we’re not testing in any meaningful way that will pick up cases that we didn’t suspect were there.”
Vivian Wang reported from Hong Kong, Donald G. McNeil Jr. and Farnaz Fassihi from New York, and Steven Lee Myers from Beijing. Marc Santora contributed reporting from London.