US public health experts said developing a vaccine for the H7N9 strain of bird flu could take "many months", as China seeks to control an outbreak which had killed 11 people by Friday.

Chinese authorities have confirmed 43 human cases of H7N9 avian influenza since announcing nearly two weeks ago that they had found the strain in humans for the first time.

Writing in the New England Journal of Medicine, Timothy Uyeki and Nancy Cox of the US Centers for Disease Control and Prevention (CDC) said worldwide efforts to develop a vaccine had started, but it would take time.

"Even if new vaccine manufacturing technologies... are utilised, the process from vaccine development to availability will probably take many months," said the article posted on the journal's website on Thursday.

China said this week it expects to have a vaccine ready in seven months.

Chinese health officials say they do not know exactly how the virus is spreading, but it is believed to be crossing to humans from birds.

The journal article said the outbreak was a "seminal event" that raised global concerns and it urged China to enhance surveillance.

"It might herald sporadic human infections from an animal source... or it might signal the start of an influenza pandemic," the article said.

Experts fear the prospect of such viruses mutating into a form easily transmissible between humans, which has the potential to trigger a pandemic.

The World Health Organization (WHO) said on Monday that there was no evidence of human-to-human transmission.

Shanghai, which has 20 confirmed cases, was the first to halt trading in live poultry and cull birds last week, followed by other cities in eastern China -- the site of the outbreak.

In Nanjing, a woman cut an official with a broken bottle as she tried to protect her chickens after the city barred residents from raising poultry at home and ordered them to cull birds, China News Service said Friday.

"I want to get bird flu, so I can infect you," the emotional woman told urban management officials seeking to kill her birds.

The United Nations' Food and Agriculture Organization (FA0) on Thursday expressed worry over the possible spread of H7N9 to China's neighbours through infected poultry.

"There is a possibility that if inadvertently or advertently somebody moves infected poultry across the border from one country to another you can have a spread of the virus," said Subhash Morzaria, Asia regional manager for the FAO's emergency centre for animal diseases.

Japan on Friday gave itself new powers aimed at curbing outbreaks of infectious diseases in people, as it nervously watches the spread of H7N9 bird flu in neighbouring China.

Under a new law, the government can strengthen quarantines at airports, vaccinate doctors and government officials, shut schools and cancel events with large numbers of people.

China has earned international approval for its transparency for H7N9, after being accused of covering up Severe Acute Respiratory Syndrome (SARS) in 2003.

"Chinese scientists are to be congratulated for the apparent speed with which the H7N9 virus was identified," said the New England Journal of Medicine article, adding they had quickly made public the genome sequences.