The so called bird flu virus may have mutated into a form that can be passed on by humans, according to the World Health Organization (WHO).

The WHO has been sounding alarm bells that a mutated form of the deadly H5N1 virus could spark a global influenza pandemic if it becomes easily transmitted from human to human.

In a statement released Monday, the WHO said a recent cluster of bird flu cases In Vietnam may be the first signal "that the virus is altering its behavior in human populations," and should be taken as a signal "to intervene quickly."

Last week, a female nurse tending to a patient with bird flu was hospitalized after showing symptoms of the disease. Previous to that, a 26-year-old male nurse who cared for the same patient tested positive for bird flu.

Authorities in Vietnam are currently testing specimens taken from the female nurse to confirm she has the disease.

Results are expected this week.