HONG KONG — Chinese authorities have killed more than 20,000 birds from a live-poultry trading zone in Shanghai after an unusual strain of bird flu that has so far killed six people in the country was found in pigeons on sale in the city, state-run media outlet Xinhua reported Friday.

Details of the slaughter of chickens, ducks, geese and pigeons come as the city prepares to temporarily close all its live poultry markets. It wasn’t clear how long the market closures — announced Friday on the Shanghai Municipal Government’s microblog account — would last.

By Friday morning, authorities in Shanghai had already closed the Huhai agricultural market, where the H7N9 avian flu virus had been found in pigeons, Xinhua reported. The virus had not previously been found in humans until a series of cases were reported in China this week.

The cull at the Shanghai poultry trading zone came as researchers in the United States said they had started work on developing a vaccine for H7N9.

The Chinese Minister of Agriculture said Thursday an analysis showed a strong genetic overlap between the strain found in the Huhai market pigeons and the one detected in infected humans.

At the Huhai market, Shanghai authorities were disinfecting the area and objects that came into contact with the birds, Xinhau reported.

Officials are trying to track where the infected pigeons came from.

A growing number of cases

A 64-year-old man died Thursday night in Huzhou, Zhejiang province, the provincial health bureau said Friday. He died hours after doctors had confirmed he had been infected with the H7N9 virus, it said.

He is one of the 14 human cases of H7N9 reported so far — all of them in the coastal area of eastern China. Authorities there began reporting the first cases on Sunday. Four of the deaths happened in Shanghai, the two others in Zhejiang.

The ages of those infected have ranged from a 4-year-old child, who was reported to be recovering, to an 83-year-old man.

No cases of human-to-human transmission of the H7N9 virus have been confirmed so far.

A person in Shanghai who developed flu symptoms after coming into close contact with a patient who died of the virus tested negative for H7N9, city authorities said.

Seeking the cause and a vaccine

“We don’t know yet where the humans got their virus from,” said Dr. Joseph Bresee, who heads the epidemiology and prevention branch in the U.S. Centers for Disease Control and Prevention’s (CDC) influenza division.

The virus has not been shown to spread easily between humans, he added.

The CDC, based in Atlanta, is working closely with Chinese authorities trying to find the source of the human infections, Bresee said.

“There are lots of things happening at CDC to prepare for this virus,” Bresee said. “State health departments are readying themselves just in case,” and researchers are working on developing a vaccine for this strain, he said.

Regional concerns

In a sign of broader regional concern about the situation, the Hospital Authority in Hong Kong said it had sent a team of six people, including experts from Hong Kong University and the Center for Health Protection, to Shanghai on Thursday.

The team’s purpose was to “learn about the experience of H7N9” in Shanghai, the authority said, adding that it was due to return late Friday.

And Japan’s National Institute of Infectious Diseases will receive samples of the virus from Chinese authorities as soon as this month, CNN affiliate NHK reported. The institute is already analyzing genetic information from the virus in order to be ready to quickly produce vaccines, if needed, NHK said.

Other strains from the H7 virus family caused previous outbreaks in poultry in countries including the Netherlands, Britain, Canada, the United States and Mexico, Malik Peiris, a professor at Hong Kong University’s School of Public Health, said earlier this week. Human infection was documented in all of those cases except the Mexican one.

The outbreak of the H7N7 strain in the Netherlands in 2003 infected 89 people, one of whom died, according to Peiris.

The better known H5N1 avian flu virus has infected more than 600 people since 2003, of which more than 370 have died, according to the World Health Organization.

In February, China reported two new human cases of H5N1 in the southern province of Guizhou, both of whom were in a critical condition, the WHO said.

A spike in H5N1 deaths, many of them children, has been reported in Cambodia, prompting concern among health authorities.

— CNN’s Jethro Mullen reported and wrote from Hong Kong. CNN’s Feng Ke and Steven Jiang in Beijing, Pamela Boykoff in Hong Kong and Miriam Falco in Atlanta contributed to this report.