Scientists in Hong Kong and the US believe the new virus, known as the "Fujian-like" strain, may have mutated in response to vaccination programmes designed to halt the disease from spreading in farm flocks.
The Fujian-like strain is now the primary variant of the fast-changing virus throughout Asia, and is quickly replacing strains that have emerged in Hong Kong and Vietnam, scientists said on Tuesday.
"Analyses [have] revealed the emergence and predominance of a previously uncharacterised H5N1 virus sublineage [Fujian-like strain] in poultry since late 2005," it said.
"The development of highly pathogenic avian H5N1 influenza viruses in poultry in Eurasia, accompanied with the increase in human infections in 2006, suggests the virus has not been effectively contained and that the pandemic threat persists," said a report by the Proceedings of the National Academy of Sciences.
"Studies suggest that H5N1 seroconversion [development of antibodies in blood serum as a result of infection or immunisation] in market poultry is low and that vaccination may have facilitated the selection of the Fujian-like sublineage."
The report, co-written by Yi Guan, of Hong Kong University's microbiology section, who has led worldwide study into the disease - said the strain had emerged in 2005, and had already spread throughout mainland China, as well as to Hong Kong, Thailand, Laos and Malaysia.
"The predominance of this virus over a large geographical region within a short period directly challenges current disease control measures," the report said.
The H5N1 strain of bird flu was first reported to have evolved into a form lethal to humans in Hong Kong in 1997, when six people died of the then mysterious disease.
A renewed outbreak in 2003 among poultry flocks in Asia caused a wave of infections that left more than 150 people dead throughout the world.
The World Health Organisation has expressed its fears that a bird flu pandemic is almost certain in the near future and could kill millions of people worldwide.