China's President Xi Jinping
Health officials raised further questions on Friday about the source of a new strain of bird flu infecting humans in China after data indicated that more than half of patients had had no contact with poultry.
The H7N9 virus has been found in 87 people, mostly in eastern China, and killed 17. But it is 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.
The WHO's China representative, Michael O'Leary, issued data on Friday showing that half of the cases analysed had had no known contact with poultry, the most obvious potential source, but he said it appeared human-to-human transmission was rare, reports Reuters.
"This is still an animal virus that occasionally infects humans," he said. "With rare exceptions, we know that people are not getting sick from other people".
Experts say it may be premature to definitely rule in or out whether people sick with the virus have been in contact with poultry, and note that contact with wild birds is even more difficult to establish.
A scientific study published last week showed the H7N9 strain was a so-called "triple reassortant" virus with a mixture of genes from three other flu strains found in birds in Asia. One of those three strains is thought to have come from a brambling, a type of small wild bird.
Ian Jones, a virologist and flu expert at Britain's Reading University, said none of the data or scientific analyses available so far have been able to pin down the exact source of the human infection or the route of transmission.
"Those are the things we need to know about," he said.
An international team of epidemiologists and other experts led by the WHO and Chinese government officials will visit live chicken markets and hospitals over the next several days in Beijing and Shanghai.