A sixth person has died of H7N9 bird flu in China, state media said Friday, after authorities slaughtered poultry in a mass cull at a Shanghai market where the virus has been detected.

The latest fatality was a 64-year-old farmer who died in Huzhou, in the eastern province of Zhejiang, local officials said according to the state Xinhua news agency.

He is thought to be among 14 previously confirmed human cases of H7N9, and is the second person from Zhejiang to die from the strain, with the other four fatalities in Shanghai, China’s commercial hub.

Authorities in the city have also said they are monitoring a person who had been in close contact with one of the dead, and who is being treated for flu-like symptoms.

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 told AFP the slaughter was completed overnight, but one of them added: “Of course, I’m worried.”

The World Health Organization (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 China Daily 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 Thursday they were liaising with domestic and international partners to develop a vaccine for the virus.

The Centers 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 told AFP.

“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 Friday that health ministry officials were meeting with agricultural personnel to draw up an action plan aimed at “preventing the spreading of the disease”.