(CNN) -- Nanjing became the third city in China to suspend live poultry trading in the wake of an unusual strain of bird flu that has so far killed six people in the country, state-run media outlet Xinhua reported Saturday
Jiangsu's provincial capital of Nanjing joined Shanghai and Zhejiang's provincial capital of Hangzhou in shuttering the markets, Xinhua said.
Four deaths occurred in Shanghai and two in Zhejiang, the agency said.
Chinese authorities have killed more than 20,000 birds from a live-poultry trading zone in Shanghai in an effort to deal with the issue.
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 H7N9 avian flu virus had been found in pigeons. The virus had not previously been found in humans until a series of cases were reported in China this week.
As of Saturday evening, China had confirmed 18 human infections of the H7N9 avian flu virus, Xinhua reported. The World Health Organization confirmed 16.
One victim, a 64-year-old man, died Thursday hours after doctors had confirmed he had been infected with the virus, Chinese officials said.
Two other men, ages 66 and 74, developed flu symptoms late last month and were diagnosed with pneumonia over the past two days, Chinese officials said. A total of 11 people who had close contacts with the two have not shown flu symptoms, officials said.
No cases of human-to-human transmission of the H7N9 virus have been confirmed.
"We don't know yet where the humans got their virus from," said Dr. Joseph Bresee, who heads the epidemiology and prevention branch of the U.S. Centers for Disease Control and Prevention's influenza division.
The virus has not been shown to spread easily between humans, he said.
The CDC, based in Atlanta, is working closely with Chinese authorities trying to find the source of the human infections.