The bird flu virus is only two mutations away from a form that can spread easily between people, sparking a pandemic in which millions could die, the UN bird flu chief said in an interview published in Portugal.

"Only two mutations are needed for it to become easily transmissible among humans," Dr David Nabarro, who heads the UN drive to contain the virus, told weekly newspaper Expresso.

"I wake up every morning thinking that today could be the day that I will see a report about a strange case of bird flu among humans," he said.

The H5N1 bird flu virus has killed tens of millions of birds since 2003 and there have been at least 165 confirmed cases of the strain spreading to humans, causing about 90 deaths, mostly in Asia.

The virus has spread from Asia to eastern Europe and Nigeria this week reported Africa's first known outbreak of the deadly strain of the disease.

Experts have long warned that the H5N1 virus could mutate into a form that is easily transmitted by humans and spark a global pandemic potentially killing millions.

Dr Nabarro said he has told governments around the world to prepare for the arrival of a human-to-human strain of the virus "as if this will happen tomorrow."

In 1918, an influenza pandemic that is believed to have originated in birds killed more than 40 million people around the world.

Subsequent pandemics in 1957 and 1968 had lower death rates but still caused widespread disruption