"What we don't know is the size of the iceberg under this tip," said the World Health Organisation's representative in China after revealing details of three families who have shown possible human-to-human transmission.
Michael O'Leary was speaking after 15 global international health experts began a weeklong mission in Beijing and Shanghai to investigate the H7N9 bird flu virus, which has killed 17 people and sickened 70 others.
"The primary focus of the investigation is to determine whether this is in fact spreading at a lower level among humans. But there is no evidence for that so far except in these very rare instances," he added.
He said one family in Shanghai had shown evidence of more than one family member being infected with the deadly strain.
In the two other clusters, one family member was infected while the other had "clinically similar and presumed H7N9".
He said investigators were trying to determine whether the family members were infected with the virus from the same source, or from each other.
China announced nearly three weeks ago that it had found the strain in people for the first time.
Almost all of the cases have occurred in Shanghai and four nearby provinces, with one incident hundreds of miles (kilometres) away in Beijing.
No vaccine to protect against the virus currently exists.
O'Leary also said investigations into the source of the virus were continuing, but it is believed to be crossing to humans from birds, prompting mass poultry culls in several cities.
He said that "more than half" of those with the virus remembered coming into contact with birds, and that health officials were continuing to examine how the others were infected.