In Jiangsu, two new cases of H7N9 infections were also reported on Friday, bringing the province's total number of cases to four.  

The new cases involve a 61-year-old woman and a 79-year-old man, both from Nanjing. The woman, a retiree who lives at home, fell ill on March 20 and was transferred to hospital for treatment on Tuesday after her situation worsened. She tested positive for the virus on Friday morning, and the man, who hails from Qinhuai district in Nanjing, tested positve for the virus on Friday morning. 

Sixty-five people who had close contact with the two in Nanjing had no flu-related symptoms, the Jiangsu Health Bureau said.

The two patients, a mainlander who came from Jiangsu and a Taiwan resident arrived from Shanghai, underwent tests for the H7N9 virus on Thursday, the news agency quoted the disease control centre official as saying.

The official said they would have the test results ready as early as Friday afternoon, the report said.

In Shanghai, the poultry cull was carried out at the Huhuai market in a western suburb of Shanghai following the discovery of the virus in pigeon samples, Xinhua reported.

Images posted on the Sina Weibo microblog by a local television reporter showed men in protective clothing and facemasks entering the market during the night, and dozens of empty birdcages stacked in the middle of the market.

On Friday morning, the entrance to the poultry section was concealed with wooden boards and sealed off with plastic tape, with a police car parked nearby and white disinfectant powder sprinkled in the street.

Two staff members at the market said the slaughter was completed overnight, but one of them added: “Of course, I’m worried.”

The World Health Organisation (WHO) has played down fears over the H7N9 strain, saying there was no evidence of human-to-human transmission, but that it was crucial to find out how the virus infects humans.

Like the H5N1 variant which typically spreads from birds to humans through direct contact, experts fear such viruses could mutate into a form easily transmissible between humans, with the potential to trigger a pandemic.

The first two deaths from the virus, which had not been seen before in humans, occurred in February but were not reported by authorities until late March. Officials said the delay in announcing the results was because it took time to determine the cause of the illness.

The state-run on Friday quoted the ministry of health in Beijing as pledging “open and transparent exchanges with the WHO and other countries and regions”.

In 2003 Chinese officials were accused of trying to cover up the outbreak of Severe Acute Respiratory Syndrome (Sars), which killed about 800 people around the world.

US health authorities said on Thursday they were liaising with domestic and international partners to develop a vaccine for the virus.

The Centres for Disease Control and Prevention, a US federal agency, said it was “gathering more information to make a knowledgeable public health risk assessment, and developing a candidate vaccine virus”.

According to the WHO, the animal source of the infection and its mode of transmission are not yet clear.

“We do not yet know enough about these infections to determine whether there is a significant risk of community spread,” the UN’s health agency said in an online H7N9 update.

Experts are concerned that the virus appears to have spread across a wide geographical area, with people sickened not only in Shanghai, but also the nearby provinces of Zhejiang, Jiangsu and Anhui.

“I am cautiously worried,” virologist John Oxford of the Queen Mary University of London said.

“If there were four cases in Shanghai, I would be much less concerned, but because it is so geographically widespread I think it is trying to tell us something.”

State broadcaster China Central Television (CCTV) reported on Friday that health ministry officials were meeting with agricultural personnel to draw up an action plan aimed at “preventing the spreading of the disease”.