However, the gene sequences of the new variant on H7N9 that is infecting humans indicated it might be better adapted than other bird flu viruses to infecting mammals, according to an article on the New England Journal of Medicine website.

That article, Human Infection with a Novel Avian-Origin Influenza A (H7N9) Virus, by Chinese scientists, was published by the renowned medical journal online on Saturday, Beijing time.

It was a joint effort by China's public health department, hospitals, universities, and research institutes, China's CDC said in a statement.

The new reassortant avian-origin influenza A (H7N9) virus was found to be associated with severe and fatal respiratory disease in human patients, the article said.

The transmission spreading dynamics of the variant H7N9 strain couldn't be underestimated and the source of infections remains unknown, it said.

As of Sunday, 60 people had been reported infected with H7N9 in China and 13 had died, according to regional health authorities.

Beijing and Henan province reported their first H7N9 human cases over the weekend, showing the virus had traveled north.

All of the previous cases were East China, including Shanghai and the adjoining provinces of Jiangsu, Anhui, and Zhejiang.

Deng Ying, director of Beijing CDC, said a risk assessment shows the capital faces challenges in containing the spread of the virus, particularly from incoming travelers infected and infected migrant birds flying northward.

A notable difficulty is that the virus only causes mild illness in animals, according to Tom Frieden, director of the US CDC.

That makes it more difficult to intervene, he said.

"Sometimes when there are things like outbreaks of H5, you have a clue because there are bird die-offs or sick flocks," he said.