BEIJING - There's no evidence a new bird flu strain is spreading easily among people in China even though there may be sporadic cases of the virus spreading to people who have close contacts with patients, the World Health Organization said Friday.

Fifteen global and Chinese health experts are on a mission in Beijing and Shanghai to learn more about the H7N9 bird flu virus that has killed 17 people and sickened 70 others, said Dr. Michael O'Leary, head of WHO's office in China.

O'Leary says a major focus is to learn how the virus infects humans. "The evidence suggests still that poultry is a vehicle for transmission but epidemiologists haven't yet been able to establish a clear and strong link," O'Leary told reporters in Beijing.

The source of the virus remains unclear because only a handful of birds — out of tens of thousands that have been tested — have been found to carry the H7N9 virus. Also, many of the patients have no reported history of contact with birds.

Still, Chinese health and agricultural authorities have closed live poultry markets and slaughtered birds as preventive measures based on suspicion that sick people had contact with infected fowl.

On Thursday, the State Forestry Administration said wild bird sales have been suspended to prevent the spread of the virus.

The team of WHO, Chinese and global experts will also study a few "clusters" of confirmed and potential infections that have emerged in the past three weeks, O'Leary said.

O'Leary maintained that there has been no evidence of sustained human-to-human transmission but that it remained unclear how in a few cases, caregivers or neighbors of patients have also become ill.

Even within the rare and isolated examples of potential clusters it is hard to determine if one person got it from another or if they were all exposed to the same source of infection, he said.

This week, Chinese health authorities confirmed that a son of an 87-year-old man in Shanghai who was the earliest known H7N9 case was also infected with the virus. The man had fallen sick in mid-February and died in early March.

At the time, two of his sons, aged 69 and 55, had also been hospitalized with pneumonia. His younger son died and no samples were available for later testing but the older son, who recovered, tested positive for the virus.

O'Leary said, however, that Chinese health authorities have closely monitored hundreds of family members, caregivers, health workers and friends who have been in contact with patients and that only a handful have signs of H7N9 infections.