A dove rests on a tree near a Chinese flag fluttering at a park on April 1, 2013, in Shanghai, China. (AP Photo/Eugene Hoshiko)

BEIJING — China announced a sixth death from a new bird flu strain Friday, while authorities in Shanghai halted the sale of live fowl and slaughtered all poultry at a market where the virus was detected in pigeons being sold for meat.

The mass bird killing is the first so far as the Chinese government responds to the H7N9 strain of bird flu, which has sickened 16 people, many critically, along the eastern seaboard in its first known infections of people. The first cases were announced Sunday, while two more were reported Friday, both retirees who were seriously ill.

Health officials believe people are contracting the virus through direct contact with infected fowl and say there has been no evidence so far that the virus is spreading easily between people. However, scientists are watching closely to see if the flu poses a substantial risk to public health or could potentially spark a global pandemic.

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The Agriculture Ministry confirmed late Thursday that the H7N9 virus had been detected in live pigeons on sale at a produce market in Shanghai. The killing of birds at the Huhuai market in Shanghai started Thursday night after the city's agricultural committee ordered it in a notice also posted on its website.

State media on Friday ran pictures of animal health officials in protective overalls and masks working through the night at the market, taking notes as they stood over piles of poultry carcasses in plastic bags. The area was guarded by police and cordoned off with plastic tape.

Experts urged Chinese health authorities to keep testing healthy birds, saying the H7N9 virus can infect birds without causing them to become ill, making it harder to detect than the H5N1 bird flu virus that is more familiar to Asian countries. H5N1 set off warnings when it began ravaging poultry across Asia in 2003 and has since killed 360 people worldwide, mostly after close contact with infected birds.

"In the past usually you would see chickens dying before any infections occurred in humans, but this time we've seen that many species of poultry actually have no apparent problems, so that makes it difficult because you lose this natural warning sign," said David Hui, an infectious diseases expert at the Chinese University of Hong Kong.

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The city of Shanghai also announced a suspension of the sale of live poultry starting Saturday, city spokesman Xu Wei said at a news conference.

Pigeon is a common type of poultry in Chinese cuisine and the birds are sold live in markets around the country. Chinese also raise pigeons as pets, but those tend to be a different type.

Hui said the pigeons were probably infected by wild or migratory birds, whose droppings can carry viruses. He said they were likely not the only species of poultry to be carrying the virus.

While health officials caution that there are no indications the virus can be transmitted from one person to another, scientists who have studied its genetic sequence said this week that the virus may have recently mutated into a form that spreads more easily to other animals, potentially posing a bigger threat to humans.

The latest death from the virus confirmed by the government Friday was a 64-year-old farmer in the eastern city of Huzhou. Authorities said Thursday the virus also killed a 48-year-old man who transported poultry for a living and a 52-year-old woman, both in Shanghai. Several among the infected are believed to have had direct contact with fowl.

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Guidelines issued Wednesday by the national health agency identify butchers, breeders and sellers of poultry, and those in the meat processing industry as at higher risk.

Experts identified the first cases on Sunday. Some of the 16 confirmed cases fell ill several weeks ago but only now are being classified as having H7N9. The official Xinhua News Agency said six cases have been confirmed in Shanghai, six in Jiangsu, three in Zhejiang and one in Anhui.

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Nurses care for victims of a Spanish influenza epidemic in tents during an outdoor fresh air cure, Lawrence, Mass., ca. 1918. (Hulton Archive/Getty Images)