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A man in the north-eastern Chinese province of Shandong has been infected by a new strain of bird flu, the first case found in the province, state news agency Xinhua reported.

The death toll from the H7N9 virus that recently emerged in humans has also risen to 22, after an 86-year-old man in eastern Zhejiang died of the sickness on Tuesday.

It is still not clear how people are becoming infected and the World Health Organisation (WHO) says there is no evidence of the most worrying scenario - sustained transmission between people.

A 36-year-old man from the city of Zaozhuang in Shandong is being treated in hospital, while two more people have been infected in eastern Zhejiang province, Xinhua said.

A total of nine people in close contact with the victim in Shandong are under observation but have shown no signs of infection, the agency said.

So far, 109 people have contracted the disease since the first deaths were reported in China last month. Authorities say many of those who becInvestigators have yet to determine human-to-human transmission of the virus.

"Investigations into the possible sources of infection and reservoirs of the virus are ongoing," the World Health Organisation said on Monday.

"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," said WHO.

The WHO's China representative, Mr Michael O'Leary, issued data last Friday showing that half of the patients analysed had no known contact with poultry, the most obvious potential source, but he said it appeared that human-to-human transmission was rare.