A seven-year-old girl in Beijing has been infected with H7N9 bird flu, the state-run news agency Xinhua reported Saturday, the first confirmed case in the Chinese capital.

Until the announcement all 43 previous human cases of the H7N9 outbreak, 11 of which had been fatal, had been confined to eastern China.

Xinhua said the latest infection was announced by local health authorities and was confirmed following a test by the Chinese Center for Disease Control and Prevention.

China officials announced nearly two weeks ago that they had found the strain in humans for the first time.

Health authorities in the country say they do not know exactly how the virus is spreading, but it is believed to be crossing to humans from birds, triggering a mass culling of poultry in several Chinese cities.

Shanghai has had 20 confirmed cases so far and was the first to halt trading in live poultry and cull birds last week, followed by other cities in eastern China.

Experts fear the prospect of such viruses mutating into a form easily transmissible between humans has the potential to trigger a pandemic.

The World Health Organization (WHO) said earlier this week that there was as yet no evidence of human-to-human transmission.

The World Organisation for Animal Health (OIE) on Thursday said H7N9 bird flu posed an "exceptional situation", explaining that the virus, while dangerous to humans, was hard to detect in the avian host.

China has said it expects to have a vaccine ready in seven months but US public health experts said developing one could take "many months".

[Image via Agence France-Presse]