Chinese officials announced nearly two weeks ago that they had found the H7N9 strain in humans for the first time, and the number of confirmed infections rose to 49 on Saturday, 11 of whom have died.
Until the girl's illness, all previous cases had been confined to eastern China, hundreds of kilometres (miles) from the capital.
Experts fear the prospect of such viruses mutating into a form easily transmissible between humans, which would have the potential to trigger a pandemic.
But the World Health Organization (WHO) said this week that there was as yet no evidence of human-to-human transmission.
One of the newly reported cases was a man in Shanghai whose wife had been confirmed with H7N9 and died earlier this month, the official Xinhua news agency reported, but it quoted health experts saying there was insufficient evidence to show he had acquired it from his wife.
Health authorities in China say they do not know exactly how the virus is spreading, but it is believed to be crossing to humans from birds, triggering mass poultry culls in several cities.
The United Nations' Food and Agriculture Organization (FA0) has said H7N9 shows "affinity" to humans while causing "very mild or no disease" in infected poultry, making finding the source of transmission more difficult.
Cheng Jun, vice-director of Ditan hospital, where the girl was being treated, told state broadcaster CCTV: "Ever since the outbreak started in Shanghai we have been making preparations."
Beijing, which has a population of more than 20 million, has already banned live poultry trading and pigeon releases, the health bureau said.
More than 500 birds were culled in the area where the family live in northeastern Beijing but samples tested negative for H7N9, Xinhua said, citing officials.
It added that the city drug administration had been ordered to stock up, including on enough Tamiflu -- the medicine most commonly used to treat bird flu in humans -- for two million people.
Users of China's hugely popular Twitter-like weibos expressed concern. One urged people to "take more rest and go out less".
"This is the first case in Beijing. It appeared in northern China. This is a bad sign," posted another.
Shanghai has had 21 confirmed cases so far and was the first place to halt live poultry trading and cull birds last week, followed by other cities in eastern China.
In 2003 Chinese authorities were accused of trying to cover up the outbreak of Severe Acute Respiratory Syndrome, which went on to kill about 800 people globally.
But China has been praised for transparency over H7N9, with the WHO saying it was pleased with the level of information sharing and US scientists congratulating it for "the apparent speed with which the H7N9 virus was identified" in a New England Journal of Medicine article.
China has said it expects to have a vaccine ready in seven months but in the article the US experts said developing one could take "many months".
US fast food giant KFC, already hit by an earlier scandal in China over antibiotics in chicken, saw March sales in the country plunge 16 percent, with parent Yum! Brands saying bird flu publicity had "a significant, negative impact".
Japan has given itself new powers aimed at curbing outbreaks of infectious diseases in people as it watches the outbreak spread in its giant neighbour, and Hong Kong has stepped up H7N9 testing in live poultry imported from mainland China.