Experts from the UN's health agency are examining whether the H7N9 bird flu virus is spreading among humans, after a cluster of cases among relatives, but downplayed fears of a pandemic Friday.

"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 as 15 global international health experts began a week-long mission in Beijing and Shanghai to investigate the H7N9 bird flu virus, which has killed 17 people and sickened 74 others.

As questions remain about the virus source and China faces global scrutiny on its handling of the disease, President Xi Jinping was also cited by state media as urging officials to do their utmost to "contain the spread" of H7N9.

O'Leary said Friday: "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 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 was "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.

Since 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 while one appeared in Beijing.

O'Leary also said investigations into the source of the virus were continuing, though the virus is believed to be crossing to humans from birds, prompting mass poultry culls in several cities.

A link to wild birds remained open to question, with a Chinese Academy of Sciences zoologist saying 15 of 16 cases it tracked occurred along a migratory route, while the State Forestry Administration said 860 wildlife samples had so far tested negative for H7N9.

O'Leary also said that over 50 percent of those with the virus remembered coming into contact with birds.

"As the investigation gets deeper we have found more than half where there is a known contact with poultry," he said.

The Chinese Center for Disease Control and Prevention had earlier said 40 percent of patients with H7N9 had not come into contact with poultry.

O'Leary stressed again that authorities had offered the WHO unrestricted access in the investigation.

In 2003 China was accused of trying to cover up the outbreak of Severe Acute Respiratory Syndrome, which went on to kill about 800 people worldwide. But it has received praise for its transparent handling of H7N9.

At a briefing Wednesday, a senior official from China's Centre for Disease Control and Prevention also stressed that the family clusters did not necessarily indicate that one relative had spread the virus to another.

"Even though there are a few family cluster cases, it does not mean the virus has the ability to effectively transmit from human to human," said Feng Zijian, according to the transcript.

A seven-year-old girl who contracted the strain left hospital on Wednesday, but since then a group of men have been stationed near her home to prevent her from straying too far, the Beijing News said on Friday.

With no vaccine for the virus for now, some people have turned to traditional Chinese medicine to guard against it, although experts warned that these might only help to treat rather than prevent the disease.

A pharmacist surnamed Zhu in Shanghai, where the largest number of cases has been found, said customers had been buying a traditional Chinese herb for colds called banlangen, which they believe gives protection from the virus.