Image caption The H5N1 virus has caused more than 360 deaths since 2003

Two men have died in the Chinese city of Shanghai, after contracting a strain of bird flu not previously known in humans, Chinese officials say.

The men, aged 27 and 87, both fell ill with the H7N9 strain in February and died some weeks later in March, Xinhua news agency reported.

A woman of 35 who caught the virus elsewhere is said to be critically ill.

It is unclear how the strain spread, but the three did not infect each other or any close contacts, officials say.

While both men who died were in Shanghai, the third victim was reported in Chuzhou in the eastern province of Anhui.

According to China's National Health and Family Planning Commission, all three became ill with coughs and fevers before developing pneumonia.

Commission experts said on Saturday the cause had been identified as H7N9, a strain of avian flu not thought to have been transmitted to humans before.

There is no vaccine against the strain, the commission said, adding it was currently testing to assess its ability to infect humans.

Another strain of bird flu, H5N1, has led to more than 360 confirmed human deaths since 2003 and the deaths of tens of millions of birds.

The World Health Organization says that most avian flu viruses do not infect humans and the majority of H5N1 cases have been associated with contact with infected poultry.