"They have not, however, had any discernible impact on the disease in humans, including its modes of transmission," the United Nations' health agency said in a statement posted on its Web site (www.who.int).

The virus, which has spread in recent months from Asia into Russia, Africa and western Europe, has so far killed more than 90 people and forced the slaughter of millions of birds as health authorities attempt to stem it.

With Western Europe on high alert - Germany, Austria, France and Italy are amongst states to report cases recently in wild birds - the WHO said the spread was cause for concern.

"The recent appearance of the virus in birds in a rapidly growing number of countries is of public health concern," it said. "It expands opportunities for human exposures and infections to occur."

The danger was greatest when the virus jumped from wild to domestic birds, which was easiest when poultry lived in close contact with humans, as in Africa and parts of Asia.

Although H5N1 remains difficult for humans to catch, scientists fear it could mutate to be easily passed from person to person and trigger a pandemic in which millions could die.

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The WHO said its statement was in answer to media speculation about the significance of mutations in the virus, with some suggesting a pandemic was growing more likely.

Although some changes had been seen in the human virus, notably in a fatal case in Turkey in January 2006, the effects were still not fully understood, it said.

However, they appeared to be one-off events.

Scientists were still in the dark about what sort of genetic changes were needed for the virus to become easily transmissible between humans, underlining the need for close surveillance and thorough investigation of all cases.

Nevertheless, there was so far "no evidence ... from any outbreak site that the virus has increased its ability to spread easily from one person to another," the WHO said.

While there was no sign yet that the virus was becoming a greater human threat, there were indications that changes in H5N1 were facilitating its spread amongst animals.

Studies had shown that the viruses were becoming progressively more lethal in experimentally infected chickens and mice and could last some three times as long - six days instead of two - in the environment at warm temperatures.

There were also indications that some of these changes were becoming fixed, making the virus genetically more stable and increasing its ability to travel and be re-introduced by migratory birds returning to breeding grounds.