The World Health Organization (WHO) on Friday confirmed an H5N1 bird flu virus mutation in Indonesia, but said the situation was not serious.

"The virus is constantly mutating, but it has not evolved into a more transmissible form," WHO spokeswoman Maria Cheng told Xinhua.

Still she warned that any mutation of the virus needed serious attention.

The world health body discovered the mutation after an investigation into a recent cluster of cases in northern Sumatra, where seven members of a single family died of the disease in May.

The report of the investigation has been given to the Indonesian government.