A new very dangerous strain of the bird flu virus that may have a higher risk of causing human death than previously identified strains has appeared in Vietnam and is spreading fast, according to state media reports.

The new strain that appeared to be a mutation of the H5N1 virus that contaminated Vietnam's poultry flocks in 2011 and forced mass culls of birds in affected areas, agriculture officials said.

The new bird flu 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 the VietnamNet online newspaper.

Experts said that the new strain appeared in July and had quickly spread through the country's northern and central regions in August.

The Animal Health Department said that outbreaks have been detected in seven provinces so far and some 180,000 birds have been destroyed.

Experts at the Central Veterinary Diagnosis Center said the virus appeared similar to other strains of bird flu but was more toxic, and researchers will test how much protection existing vaccines for human offer. Some investigators suggested that the new strain of bird fly resulted from the widespread smuggling of "disqualified chickens" from China into northern parts of Vietnam.

"Wherever Chinese disqualified chickens come, the new virus appears. It attacked both chickens and ducks," Thanh Son Nguyen of the Animal Husbandry Agency said, according to VietnamNet. So far, two people have died this year from the infectious disease, before the new strain was identified.

Vietnam has had 59 deaths from the avian influenza virus since 2003, the highest numbers of fatalities from bird flu in Southeast Asia, according to the World Health Organization. Bird flu has killed more than 330 people worldwide, and scientists fear that the virus could mutate into a more deadly form that can be easily transmitted between humans.