BEIJING -- China is investigating four possible cases of human-to-human transmission of a deadly bird flu that has killed 17 people, but so far there has been "no sustained" evidence of transmission between people, the World Health Organization said Thursday.
Three families in Shanghai and two young children in Beijing were being examined as possible examples of human-to-human transmission, Gregory Hartl, the spokesman for W.H.O. in Geneva, said in a telephone interview.
"Even if two family members are positive, it is not necessarily the case they got it from each other," Mr. Hartl said. "They may have gotten it from the same bird."
As investigators looked at the possibility of human transmission, there was mounting concern that the new virus, known as H7N9, may not originate in poultry but in other animals, he said.
To that end, a team of international influenza experts from the agency's headquarters in Geneva and a regional office in Manila and scientists from the United States Centers for Disease Control who were invited by China to help investigate the virus, arrived in Beijing on Thursday. The experts would be looking at possible sources for the virus other than birds, Mr. Hartl said.
A Chinese expert on the disease, Feng Zijian, the director of the health emergency center at the Chinese Center for Disease Control and Prevention, said on Wednesday that an estimated 40 percent of people infected with the virus said they never had contact with poultry.
Mr. Hartl concurred with Mr. Feng about the surprisingly low incidence of infected people who had no contact with birds. "It is not clear all cases so far have had contact with poultry," Mr. Hartl said.
Because it seemed possible that the virus originated in animals other than poultry, the international investigating team would be casting a wide net for possible sources, Mr. Hartl said.
Seventeen people have died since China told the World Health Organization on March 3 of the bird flu outbreak, according to China's state-run news agency, Xinhua. There were 83 cases of infection, the news agency said.
Even as the international investigators would be seeking other sources of infection, China's agricultural authorities were insisting the H7N9 virus was still confined to live poultry markets. The news agency said that 47,801 samples had been collected from 1,000 poultry markets, habitats and farms from across China, and that agricultural authorities said that 39 tested positive for H7N9.
Mr. Hartl noted that the percentage of positives was very low.
Early suspicions that pigs might be the carrier of the virus have not been confirmed, Mr. Hartl said. Pigs were tested soon after the outbreak was announced, he said, and there were no positive results.
The Chinese authorities had informed the W.H.O. about three families in Shanghai where more than one person was infected with the virus, Mr. Hartl said. In two, two people were infected, he said, and in the other, three were infected. In that case, an 87-year-old man and his 55-year-old son died, and his other son, 69, was sick, Mr. Hartl said.
The two children infected in Beijing, a boy and a girl, were neighbors and often played together, Mr. Hartl said. It is possible, he said, that they may have picked up the virus from the same infected bird.
In a news conference on Wednesday, Mr. Feng played down the possibility of "effective" human-to-human transmission.
"Effective human-to-human transmission is the case when a disease becomes a human flu virus, as seen in the case of H1N1, where groups of people would be infected at once, such as in schools and communities," Mr. Feng said. "Effective human-to-human transmission means one patient could infect many and the virus continues to pass on to first, second and third patients. Effective human-to-human transmission has a clear chain of infection."
The H1N1 virus was a new flu virus strain that caused a world wide pandemic in humans from June 2009 to August 2010.
There was "currently no evidence showing that H7N9 carries continuous infecting power," Mr. Feng said.
Mia Li contributed research.
Correction: April 18, 2013, Thursday
This article has been revised to reflect the following correction: An earlier version of this article misstated the name of a new strain of avian flu. It is H7N9, not H7H9.
It also incorrectly described the two infected children in Beijing as boys. One is a boy and one is a girl.
This article originally appeared in The New York Times.