"As avian influenza viruses become more active at cooler temperature, further poultry outbreaks, possibly accompanied by sporadic human cases, can be anticipated," the UN health agency said in a statement dated Dec. 30.

"Poultry marketing, transportation and consumption increase in Vietnam with the approach of the Lunar New Year in early February. These activities create conditions favouring the spread of the poultry outbreaks and call for heightened control measures," WHO said.

Doctors said on Friday the girl´s condition remained stable after she was hospitalised with a high fever on Sunday from the southern province of Tay Ninh, bordering Cambodia.

"She is still breathing with a respirator, but her condition is not worsening which is a good signal," doctor Tran Tinh Hien, deputy director of the Ho Chi Minh City Hospital for Tropical Diseases, told Reuters.

The family of the girl had killed and eaten infected chickens before she fell ill, state media has said.

Vietnam said earlier in December that bird flu had been detected in six southern provinces and more than 12,000 birds have been culled since then in the Mekong river delta to prevent the virus spreading and infecting people.

This week nearly 2,000 infected chickens and geese were culled in An Giang and Tien Giang provinces.

A total of 32 people -- 20 in Vietnam and 12 in Thailand -- have died of bird flu this year.

State media said on on Friday that fresh outbreaks of the virus were found this week in the northern province of Nam Dinh, the first time it has been detected in the north of the country since September.

Temperatures in the north are now at the coldest for winter, which lasts until late February.

Health workers have culled more than 900 poultry in Nam Dinh but no human infections were detected, said state-run Voice of Vietnam radio.