A new highly-toxic strain of the potentially deadly bird flu virus has appeared in Vietnam and is spreading fast, according to state media reports.

The strain appeared to be a mutation of the H5N1 virus which swept through the country's poultry flocks last year, forcing mass culls of birds in affected areas, according to agriculture officials.

The new virus "is quickly spreading and this is the big concern of the government", Deputy Minister of Agriculture and Rural Development Diep Kinh Tan said, according to a Thursday report in the VietnamNet online newspaper.

Experts cited in the report said the new virus appeared in July and had spread through Vietnam's northern and central regions in August.

Outbreaks have been detected in six provinces so far and some 180,000 birds have been culled, the Animal Health department said.

The Central Veterinary Diagnosis Centre said the virus appeared similar to the standard strains of bird flu but was more toxic.

The centre will test how much protection existing vaccines for humans offer, the report said.

Some experts suggested that the new strain resulted from widespread smuggling of poultry from China into the northern parts of Vietnam.

Two people have died this year from the virulent disease -- but long before the new strain was identified.

According to the World Health Organisation, Vietnam has recorded one of the highest numbers of fatalities from bird flu in southeast Asia, with at least 59 deaths since 2003.

The avian influenza virus has killed more than 330 people around the world, and scientists fear it could mutate into a form readily transmissible between humans, with the potential to cause millions of deaths.