Bird flu has struck two poultry flocks in northern Vietnam, a newspaper reported Sunday, after authorities warned that the killer virus could resurface strongly in the cold and wet winter months.

The virulent H5N1 viral strain killed five people in Vietnam early this year, but no new human deaths have been reported since March.

Last week the virus struck again in Thai Nguyen city, 80 kilometres (50 miles) north of the capital Hanoi, where it was found in several dead ducks on one farm and in dead chicken on another, the Tuoi Tre daily said.

Veterinary authorities culled 100 ducks and more than 4,000 chicken, said the report quoting local animal health officials.

Northern Vietnam has in recent days been gripped by a cold snap that favours the spread of respiratory diseases because the virus survives longer in the cold and people spend more time close together inside their homes.

The Tet lunar New Year festival in January is also considered a high-risk time for the spread of bird flu because the slaughter, trade and consumption of poultry, and smuggling of birds from neighbouring China, rises sharply.

Vietnam's health ministry last week issued a nationwide bird flu alert, urging cities and provinces to step up detection and epidemic control measures.

Communist Vietnam, once the nation worst hit by the H5N1 virus, contained earlier outbreaks through mass poultry vaccination campaigns, the destruction of millions of birds and public education initiatives.