(Updates with human transmission ruled out)

GENEVA, Apr 3 (Reuters) - Egyptian health authorities excluded the possibility of human to human transmission in the case of a brother and sister with bird flu, 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.

"We have heard from the Ministry of Health that human to human transmission has been ruled out," WHO spokesman Greg Hartl told Reuters.

Both of the children had been exposed to poultry infected with the H5N1 virus, the most common way in which bird flu has been spreading.

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.

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