The H7N9 bird flu virus that has killed 27 people in China might have originated from chicken and duck influenza viruses, a study suggests.

Chinese researchers have published a detailed genetic analysis of the H7N9 virus, which was identified on March 30.

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A breeder covers his face as he sits behind chickens, in Yuxin township, Zhejiang province, last month. Poultry-to-person virus transmission in China needs to be watched carefully, researchers say. (William Hong/Reuters)

Results of the study appear in this week's issue of the medical journal The Lancet.

Researchers examined genomic sequences of the virus and ecological information such as the birds' migratory paths to extrapolate its potential family tree and possible routes.

Scientists are watching the virus closely to see if it could spark a global pandemic.

The World Health Organization said Thursday there is no evidence of sustained human-to-human transmission, meaning it doesn't spread easily from human to human.

"Investigations into the possible sources of infection and reservoirs of the virus are ongoing," WHO said in a statement.  "Until the source of infection has been identified and controlled, it is expected that there will be further cases of human infection with the virus."

The study is part of the investigation into the potential animal reservoirs of H7N9, such as the "intermediate hosts" like birds or mammals that pass it on to humans.

"The novel avian influenza A H7N9 virus might have evolved from at least four origins," Prof. George Gao of the Chinese Center for Disease Control and Prevention in Beijing and his co-authors concluded.

"Unknown intermediate hosts involved might be implicated, extensive global surveillance is needed, and domestic-poultry-to-person transmission should be closely watched in the future."

The researchers said the evidence points to H7N9 gaining genes for its surface proteins from duck bird flu viruses while the virus's internal genes seem to have come from chicken viruses.