The World Health Organization (WHO) has yet to find evidence that the H7N9 avian influenza has become human-to-human transmissible, WHO representative Dr. Michael O'Leary said Monday.

"Although we do not know the source of infection, at this time there is no evidence of sustained human-to-human transmission," O'Leary said at a news conference held by China's National Health and Family Planning Commission (NHFPC).

Twenty-one cases of H7N9 have been reported in China thus far, appearing in Shanghai and three east China provinces.

Six of the cases have resulted in death, including the deaths of a man and one of his sons in Shanghai. The man's second son was also infected but has since recovered.

"The family cluster raises the possibility of human-to-human transmission, but two of the cases in that cluster have not been laboratory confirmed," O'Leary said.

"There is no other evidence pointing toward sustained transmission among people," he added.

O'Leary also responded to an alleged link between the cases and the large number of dead pigs that were recently found dumped in rivers in east China.

The pigs' connection with the human infections has been ruled out, O'Leary said.

In some of the H7N9 cases, the infected people were found to have had contact with animals, he said, citing the appearance of the virus in pigeons in a Shanghai marketplace.

These cases indicate that the virus may have crossed from animals to humans, he said, adding that a related investigation is under way.

WHO has been closely collaborating with the NHFPC and other government departments, he said.

WHO does not deem it necessary at present to introduce screening measures at ports of entry or adopt any restrictive measures on tourism and trade, O'Leary said.

He said the public should maintain good health habits and only eat properly cooked meat.