The toll from the disease which has hit 10 Asian nations and killed five in Thailand and 14 in Vietnam continued to mount however, with Thailand announcing a sixth person had been infected.

The deaths of two Vietnamese sisters whose brother was also killed by the virus has been investigated as the first possible case of human-to-human transmission of the disease -- a potentially catastrophic development.

However, the WHO said in a statement that test results on the second woman were reassuring and reflected the findings of tests carried out on the first sister last week.

"Virus genetic material from this woman, as for the other case in this cluster, is of avian origin and contains no human influenza genes," it said.

The UN health agency has warned that the deadly H5N1 strain of bird flu could kill millions across the globe if it combines with a human influenza virus to create a new, highly contagious strain transmissible among humans.

Earlier this month the WHO said human-to-human transmission was a "possible explanation" in the case of the Vietnamese women, who were part of a cluster of four cases which included their brother and his wife.

The WHO representative in Hanoi, Pascale Brudon, said Thursday that the test results did not exclude a very limited human-to-human transmission of the virus.

However she added: "If any transmission took place it did not spread outside this small cluster of cases, so it is of less concern. But the epidemiological tests will never answer all our questions."

The deadly H5N1 strain has emerged in Cambodia, China, Indonesia, Japan, Laos and South Korea. Pakistan and Taiwan have announced weaker strains, as has the United States where bird flu has been found in the state of Delaware.