Story highlights 13 dead from a total of 60 cases of bird flu in China, authorities say

The virus is normally found in birds

H7N9 was never known to infect people until last month

Cases of the virus have now spread to central and northern China

Two more people infected with a rare strain of bird flu in China died over the weekend, as the number of human cases of H7N9 climbed to 60, state media reported Sunday.

The two deaths, both in Shanghai, takes the death toll to 13, the Xinhua news agency said.

Shanghai's health authorities have now reported 24 cases, after three men were diagnosed with H7N9 on Saturday.

The virus also appears to have spread beyond eastern China for the first time. Two new cases were reported in central Henan Province on Sunday morning, while a child in Beijing in the north tested positive on Saturday.

The H7N9 strain is normally found in birds and was never known to infect people until last month.

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The World Health Organization (WHO) said China had been infected with a new variation of bird flu. The agency said it continued to look for the source of the infection.

"Investigations into the possible sources of infection and reservoirs of the virus are ongoing," the organization announced on Saturday. "Until the source of infection has been identified, it is expected that there will be further cases of human infection with the virus in China."

So far, WHO said, there is no evidence of ongoing human-to-human transmission.

A possible source for the infections is poultry markets, which have become the focus of investigation by China's health ministry and WHO.

Several cities in eastern China have suspended trading in live poultry in an effort to contain the problem.

Chinese scientists said the H7N9 virus probably came from migratory birds from East Asia that mixed with domestic fowl around Shanghai.

The new variation of bird flu has genetic characteristics that make it well-adapted to infect humans, the New England Journal of Medicine said.