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 Organization (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 analyzed had had no known contact with poultry, the most obvious potential source, but he said it appeared human-to-human transmission was rare.
"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.
Some bird samples have tested positive and China has culled thousands of birds and shut down some live poultry markets.
A WHO spokesman in Geneva, Glenn Thomas, said that "evidence suggests that poultry is a vehicle of transmission, but epidemiologists haven't yet been able to establish a strong and clear link".
"We also know that, perhaps with rare exceptions, people are not getting sick from other people."
The official China Daily newspaper quoted an investigator at the Chinese Academy of Sciences as saying the H7N9 outbreak may linked to the migration of wild birds.
"The infection time and route coincided with the migration of water birds," He Hongxuan told the newspaper.
Chinese President Xi Jinping on Friday called for efforts to strictly control sources of infection, boost checks of live poultry and release accurate information.
He said researchers should work to accelerate development of vaccines while local authorities should strengthen disease control, according to a statement released by the Central Committee of the Chinese Communist Party, of which he is the secretary.
China has warned that there could be more infections to come. Many of the 87 cases, and 11 of the deaths, have occurred in the commercial hub of Shanghai.
China's poultry sector has recorded losses of more than $1.6 billion since reports of the new flu surfaced two weeks ago, an industry official said this week.