The death toll from H7N9 bird flu in China reached 10 on Thursday with another victim in Shanghai, as cities banned people from raising chickens at home to try to contain the outbreak.
China has confirmed 38 human cases of H7N9 avian influenza after announcing on March 31 that it had found the strain in people for the first time.
One person, a young boy in Shanghai, has been discharged from hospital after recovering but the city reported the death of a 74-year-old retired man on Thursday.
Chinese authorities say they do not know how the virus is spreading, but it is believed to be crossing to humans from birds.
Experts fear the prospect of such viruses mutating into a form easily transmissible between humans has the potential to trigger a pandemic.
The World Health Organization (WHO) said earlier this week that there was no evidence of human-to-human transmission.
The United Nations' Food and Agriculture Organization (FA0) said Thursday that H7N9 showed "affinity" to humans while being harder to detect in birds, which made finding the source of transmission more difficult.
"This new virus shows very strong affinity to humans and infects poultry but causes very mild or no disease," said Subhash Morzaria, Asia regional manager for the FAO's emergency centre for animal diseases.
"So from a perspective of understanding the transmission, we have a problem because these poultry are silent carriers of the virus," he told a news conference in Bangkok.
The prestigious Chinese Academy of Sciences said Wednesday H7N9 had probably originated from migratory birds from East Asia mixing with domestic fowl in China's Yangtze River delta region—the site of the current outbreak.
Five more markets across eastern China had found H7N9 in samples from chickens and ducks, the Ministry of Agriculture said Wednesday.
Nanjing city had barred urban residents from raising poultry and livestock on their property, asking them to cull their own animals and fining them up to 50 yuan ($8) for violations, the China Daily newspaper reported on Thursday.
"People from the neighbourhood committee came to my house, asking me to kill the chickens I have been raising, but I really didn't have the courage," a Nanjing resident using the name Niuye Buniuma said on a microblog.
Shanghai said it would enforce a long-standing ban against residents raising poultry and rabbits for meat, giving a telephone hotline for people to inform on their neighbours for violating the rules.
"Many citizens have expressed the reaction that raising chickens, ducks and pets in their neighbourhoods might bring danger to the surroundings," a Shanghai government statement said.
Shanghai last week suspended trading in live poultry and shut markets in a bid to curb the outbreak while Nanjing did the same, followed by other cities.
The Shanghai government said it would offer compensation for losses in the poultry industry, including payments of two or three yuan ($0.32 or $0.48) for chickens from smaller farms.
China's State Council, or cabinet, Wednesday urged "efficiency and transparency" in tackling the outbreak as the government tries to show openness after being accused of covering up Severe Acute Respiratory Syndrome (SARS) in 2003.
There are signs the H7N9 bird flu outbreak is hurting business in some sectors, including restaurants.
US fast food giant KFC, already hit by an earlier scandal in China over antibiotics in chicken, saw March sales in the country plunge 16 percent.
"Publicity associated with avian flu in China has had a significant, negative impact on KFC sales,' parent Yum! Brands said Wednesday.
Hong Kong is stepping up rapid testing for H7N9 in all live poultry imported from mainland China.