GENEVA, Apr 3 (Reuters) - Human to human transmission of the H5N1 virus between a brother and sister in Egypt cannot be ruled out yet, although both siblings seem to have been exposed to sick birds, the World Health Organisation (WHO) said on Tuesday.

A four-year-old boy, from Qena province around 670 km (400 miles) south of Cairo, was among three human cases announced by the health ministry at the weekend. His six-year-old sister was one of two children diagnosed with the virus late last week.

"Human to human transmisssion cannot yet be ruled out. We are continuing investigations," WHO spokesman Gregory Hartl said in Geneva. "But we know all the children had exposure to sick or dead birds."

In all, five Egyptian children have been reported as being in hospital in stable condition. "Egypt has an extremely good record of child survival of H5N1," Hartl added.

The highly pathogenic H5N1 virus is not easily transmissible between people, although there has been evidence of several clusters involving human to human transmission over the past three years, according to the WHO.

Experts fear that the virus will mutate or combine with the highly contagious seasonal influenza virus and spark a deadly pandemic which could circle the globe and kill millions.

Egypt has the highest number of confirmed human bird flu cases outside Asia. Of the 32 confirmed cases in the country to date, 13 have been fatal.