An employee wearing a protection suit sprays disinfectant on chickens at a poultry market in China, April 5, 2013. Chinese authorities were slaughtering birds at a poultry market in Shanghai as the death toll from a new strain of bird flu mounted to six on Friday. (JIANAN YU/REUTERS)

The death toll from a new strain of bird flu rose to six in China on Thursday as scientists at the U.S. Centers for Disease Control and Prevention and around the world stepped up efforts to determine its pandemic risk.

This is the first time the H7N9 virus has been detected in humans, but there is no evidence that the strain is transmitted from human to human, officials said. At least 14 people in China have been confirmed to have H7N9, all in the eastern part of the country. According to the World Health Organization, three of the most recent fatal cases involved men: a 38-year-old from Zhejiang province, in eastern China, who became sick March 7; a 64-year-old, also from Zhejiang, who became ill March 28; and a 48-year-old from Shanghai who also became sick March 28.

Chinese authorities confirmed the sixth death on Friday, hours after the CDC reported it.

Shanghai officials closed the Huhai agricultural market on Thursday and began slaughtering birds there after discovering the new virus strain in a market pigeon. By late Friday, they had killed more than 20,000 birds at the market. Officials announced that all live poultry markets in the city would be closed beginning on Saturday.

Meanwhile, a person who had contact with one of the victims in Shanghai was in quarantine after developing flu-like symptoms, according to the Shanghai Municipal Health and Family Planning Commission.

“These deaths don’t have any links or common exposures that we know of,” Joe Bresee, chief of epidemiology for CDC’s Influenza Division, said in an interview Thursday. Chinese health authorities are monitoring more than 400 people who were in close contact with people in which the strain was confirmed. None of these people are sick, he said.

“Clearly, if there is evidence that the virus spread from human to human, that becomes a game-changing event,” Bresee said.

Officials in Hong Kong and Japan said they are taking precautions at airports. Posters and personnel are warning passengers to seek medical attention if they suspect they have bird flu. Vietnam banned all poultry products from China this week, and Taiwan set up a monitoring group.

After Chinese authorities revealed the cases last weekend, officials quickly posted the genetic sequence data from the first three cases on a public database, Bresee said. The virus’s genetic sequence shows that it’s a combination of an H7N9 virus that circulates in birds and an H9N2 pathogen, scientists have said.

The CDC is developing a kit that could be sent to other countries to allow public health officials to test for the virus, Bresee said. The agency also is using the genetic information to grow a seed virus for potential production of a vaccine, he added. That has become a routine procedure for the CDC in the past five to six years.

Other strains of bird flu, such as H5N1, have been circulating for years and can be transmitted from bird to bird, and bird to human, but not generally from human to human.

The H1N1 virus responsible for the 2009-2010 swine flu pandemic originated in pigs, then mixed with human and avian viruses, touching off the first global influenza outbreak in more than 40 years. It killed 151,700 to 575,400 people in the first year, with a disproportionate number of deaths in Southeast Asia and Africa, according to the CDC.

It’s possible that the new bird flu virus may persist in animals and occasionally occur in humans, but not develop the capacity to spread in humans, Bresee said.

Researchers don’t know the source of the infection. The H7N9 virus is usually present in poultry and wild bird populations and doesn’t generally make them very sick, Bresee said.

Chinese officials have tried to dispel rumors that the new bird flu strain was related to the recent scandal triggered by the discovery of more than 15,000 dead pigs in Shanghai area rivers. State media reported that health officials tested 34 samples from pig carcasses and found no traces of bird flu.

“We don’t know that it’s not pigs,” Bresee said. “We have seen evidence from the genetic sequence that at some point, it passed through poultry. Could it be chickens? Could it be pigs? I don’t know.”

Scientists also don’t know whether the virus is resistant to drugs or whether it has mutations that would give it greater ability to transmit and cause severe disease.

Some Chinese newspapers have questioned why it took so long for the government to announce the new cases, especially because two victims fell ill in February. The government has said it needed time to correctly identify the virus.

During the 2002 and 2003 outbreak of severe acute respiratory syndrome, authorities initially tried to cover up an epidemic that emerged in China and killed nearly 10 percent of the 8,098 people infected worldwide, according to the World Health Organization.

Bresee, who worked in Hong Kong during the SARS epidemic, praised Chinese officials for how quickly they have been sharing information about the new bird flu strain.

William Wan in Beijing contributed to this report.