Tests have yielded no evidence so far that the bird flu virus is mutating and becoming easier to transmit between humans, the World Health Organisation said on Thursday.

In May the WHO warned that the virus in Vietnam, the country with the highest number of cases, could be changing and becoming easier to pass on.

Such a mutation could herald the start of a long-predicted international flu pandemic capable of killing millions of people around the world.

But the Geneva-based body said in a brief statement that laboratory and epidemiological examinations of recent Vietnamese cases, carried out by an international team, had revealed no change to the virus.

“We did not find evidence to substantiate what was suggested in Manila,” said WHO spokeswoman Maria Cheng, referring to the meeting in the Philippines where the concerns first surfaced.

“They (the investigators) have not found any change in the virus,” she said, but she added further tests would be needed.

“While these results are reassuring, further testing of clinical specimens will continue over the next few weeks to provide the most reliable possible foundation for risk assessment,” the WHO statement said.

Cheng said that the tests would be carried out in laboratories in the United States, Britain and Japan.