President Bush issued a directive on Friday allowing authorities to detain or isolate any passenger suspected of having Avian flu when arriving in the United States aboard an international flight.

The H5N1 bird flu virus has killed 49 people in Asia since 2003 - 35 in Vietnam, 12 in Thailand and 2 Cambodians - while millions of birds have been slaughtered to contain the disease.

Even so, the World Health Organization has said it had seen no evidence so far to suggest the bird flu virus was changing into a form that could be easily transmitted from one human to another.

White House spokesman Trent Duffy called the directive a precautionary measure.

“There’s no evidence of any risk of avian flu to the American people at this time. But in an abundance of caution, this is a pragmatic step to ensure the government has the authority it needs to protect the American people as best it can,” Duffy said.

The Bush order added pandemic influenza to the list of diseases for which quarantine is authorized. Pandemic flu is considered a novel or re-emergent strain to which there is little or no population immunity.

Under the directive, the Health and Human Services Department is given legal authority to detain or isolate any passenger suspected of having the avian flu to prevent the person from infecting others.

Quarantine and isolation were last used during the outbreak of SARS in 2003.