Two Chinese men have died from a lesser-known type of bird flu in the first known human deaths from the strain.
Health experts said it was not clear how they were infected in the Shanghai area but there was no evidence of human-to-human transmission.
A third person, a woman in the nearby province of Anhui, also contracted the H7N9 strain and was in critical condition.
There was no sign that any of the three, who were infected over the past two months, had contracted the disease from each other, and no sign of infection in the 88 people who had closest contact with them, China's state medical agency said.
H7N9 bird flu is considered a low pathogenic strain that cannot easily be contracted by humans. The overwhelming majority of human deaths from bird flu have been caused by the more virulent H5N1, which devastated poultry stocks across Asia in 2003.
The World Health Organisation is "closely monitoring the situation" in China. "There is apparently no evidence of human-to-human transmission, and transmission of the virus appears to be inefficient, therefore the risk to public health would appear to be low," a spokesman said.
Scientists have been closely monitoring the H5N1 strain of the virus, fearing that it could mutate into a form that spreads easily among people, potentially sparking a pandemic. So far, most human cases have been connected to contact with infected birds.
It was unclear how the three patients became infected, the health agency said. It sought to calm fears about the virus but provided few details about each case. Authorities have not described the patients' occupations or said whether they had come into contact with birds or other animals.
More than 16,000 pig carcasses were fished out of the river system that supplies some of Shanghai's water supply in March, apparently dumped by farmers after they fell ill, and some observers have wondered whether there might be a link between the pigs and the bird flu deaths.
Many epidemiologists regard densely populated parts of China and Southeast Asia, where farmers often live in close quarters with pigs and poultry, as regions where conditions are ideal for nurturing infectious diseases that jump from animals to humans. An earlier deadly outbreak of SARS, or severe acute respiratory syndrome, was linked to wild animals that infected animals, which in turn infected people in that region.