Two people in China have died after being infected with H7N9 avian influenza -- a sub-type that had not previously been transmitted to humans -- the government said Sunday.

An 87-year-old man in Shanghai fell ill on February 19 and died on March 4, while a man aged 27 in the city got sick on February 27 and died on March 10, the National Health and Family Planning Commission said on its website.

A 35-year-old woman in the eastern province of Anhui became ill on March 9 and is in critical condition, it said.

All three initially suffered fever and coughs but later developed severe pneumonia and problems with breathing, according to the announcement.

The three cases were confirmed Saturday to be infections with the H7N9 sub-strain after the Chinese Center for Disease Control and Preservation had separated it from body samples the day before, the commission said.

The route of infection remained unknown although nothing unusual was detected among 88 people close to the victims, the commission said.

The H7N9 strain had not been transmitted to humans before, it added.

In a separate release, the commission said there are currently no vaccines against H7N9.

It also explained that the delay in announcing the results was because it took time to determine the cause of the illnesses.

China is considered one of the nations most at risk from bird flu because it has the world's biggest poultry population and many chickens in rural areas are kept close to humans.

China has in the past been accused of covering up the extent of bird flu outbreaks, exacerbating fears when new cases are reported.

The World Health Organisation says more than 360 confirmed human deaths from the H5N1 strain of avian influenza have been reported to it globally from 2003 until March 12 this year.

It says other sub-types have infected humans, including H7N7 and H9N2, and some of these infections led to deaths.