They were among the first people to catch the virus, which was previously only found in birds.

Experts are now studying how H9N2 is transmitted.

However, they say it is milder than the H5N1 virus which killed six people in 1997.

Chief of Epidemiology of the Influenza Branch of the Centres for Disease Control and Prevention in Atlanta Dr Keiji Fukuda said he would not rule out an epidemic.

'The chances of that [occurring] immediately are relatively low, but again we are always mindful of the fact that the virus can change or re-assort itself and become easily transmitted.

'Having found two human cases, new cases will not be unexpected.

'I think that is one of the lessons learned from the H5N1.' Experts are still grappling with whether the H9N2 poses a threat to humans, Dr Fukuda said.

'We don't know how quickly it will spread, how widespread it is among birds and or humans.' Chickens, duck, geese and pigeons are known to carry the virus but Dr Fukuda said that with unconfirmed reports of the infection, it was difficult to trace the exact source of the virus.

He said the ultimate source of the virus could be traced to wild birds which infect the poultry population.

The four-year-old girl, found with H9N2 on February 28 had visited her grandfather, who kept livestock, but relatives did not see any contact with chicken, health department consultant of community medicine Dr Mak Kwok-hang said.

So far, none of the girls' relatives has been confirmed with the virus although some had complained of sore threats - a symptom suffered by both girls.

Dr Mak said it was safe to eat properly cooked poultry, because bird flu was not contracted through food.

The department was well equipped to deal with the virus when the flu season traditionally peaked in July, Dr Mak said.