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Possible human-to-human transmission of the H7N9 bird flu, that has killed 17 people in China, is being investigated. There is currently "no sustained" evidence of transmission between people according to the World Health Organization (WHO).

Two young boys in Beijing and three families in Shanghai were being examined as possible cases of human-to-human transmission, Gregory Hartl, the spokesman for WHO in Geneva told the New York Times.

 "Even if two family members are positive, it is not necessarily the case they got it from each other. They may have gotten it from the same bird," Hartl said.

The possibility of human transmission has investigators growing concerned that the new virus, H7N9, may actually originate in other animals and environmental sources rather than birds, the spokesman said.

International influenza experts from WHO and scientists from the United States Centers for Disease Control (CDC) were invited to China to help investigate the virus. They arrived in Beijing on Thursday to look for possible sources for the virus other than birds.

An estimated 40 percent of people infected with the virus said they never had contact with poultry, said Feng Zijian, director of the health emergency center at the Chinese Center for Disease Control and Prevention.

 "It is not clear all cases so far have had contact with poultry," Hartl concurred with Zijian.

The possibility that the virus originated in other animals or from an environmental source means the international team of investigators will be casting a wide net for possible sources, Hartl said.

Since the H7N9 outbreak was reported to WHO on March 3, there have been 83 cases of infection and 17 deaths, according to China's state-run news agency, Xinhua.

China's agricultural authorities still insisted that the virus was confined to live poultry markets. The news agency said the 47,801 samples had been collected from 1,000 poultry markets, habitats and farms across China.

According to agricultural authorities, only 39 samples tested positive for H7N9. The number of birds that tested positive from such a large sample is actually quite low, said Hartl.

Pigs had been tested soon after the outbreak following an early suspicion they may have been the carrier, but there were no positive results.

Chinese authorities informed WHO of three families in Shanghai where more than one person was infected, Hartl said. In two of the families, two members were infected. In the other family, three people were infected-the father, age 87, and his son, age 55, both died. The other son, 69, was sick but is still alive, according to Hartl.

Two boys in Beijing who were neighbors and played together often were also infected. In this case, it is possible that the boys could have picked up the virus from the same infected bird, Hartl said.

During a news conference on Wednesday, 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," 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 worldwide pandemic in humans from June 2009 to August 2010.

There is "currently no evidence showing that H7N9 carries continuous infecting power," Feng said.