GENEVA (AFP) - The World Health Organisation on Friday ruled out any mutation of the potentially fatal H5N1 strain of the bird flu virus after a case of inter-human transmission of bird flu may have been detected in Pakistan.

"There is no suggestion that the virus has changed into a form that poses a broader risk," WHO spokesman John Rainford told AFP. "If that had been the case, we would have witnessed more cases of human transmission."

Rainford said that the genetic sequencing of the virus involved in the latest case was being continued.

Laboratory tests have already established that the Pakistani man had been infected with H5N1, even though he had not been in contact with contaminated poultry.

"Because we have an individual not directly exposed to sick birds suggests a limited human-to-human transmission," Rainford had told AFP on Thursday.

According to results of investigations conducted by Pakistan's health ministry, the H5N1-positive case was a 25-year-old man from the Peshawar area who developed febrile respiratory illness on November 21, was hospitalized on November 23, and died on November 28.

He is the third of four brothers who developed proven or suspected pneumonia with illness onset dates between October 29 and November 21, said Rainford.

The brothers provided care for one another and had close personal contact in both the home and hospital.

Human-to-human contamination has been reported in Cambodia, Indonesia and Vietnam in recent months, but has not spread beyond a single person. A suspected case in China was denied by the authorities there.

Experts fear that if the H5N1 strain mutates into a highly contagious form, it would provoke a pandemic on the scale of the 1918 Spanish flu outbreak, which claimed tens of millions of lives.

The WHO team was sent to Pakistan after the health ministry announced the death of a man who was one of six people infected with the H5N1 strain in North West Frontier Province along the Afghanistan border.

A brother of the victim also died before being tested for the virus. Both had worked on a cull of infected poultry.

"Wherever you have poultry outbreaks, you have to be on guard for human cases," Rainford cautioned.