Bird flu outbreaks that have killed seven people in several countries so far this year show the virus remains a threat to humans, the World Health Organisation (WHO) said Wednesday.

"The newly confirmed human and poultry cases of avian influenza this year are a reminder that the virus poses a real and continuous threat to human health," the WHO said in a statement.

One danger is that bird flu, also known as H5N1, may mutate, warned Takeshia Sakai, the WHO's regional adviser for communicable diseases.

"The influenza virus is unpredictable," Sakai said. "There is a constant risk that the H5N1 virus will combine with another strain of influenza."

So far this year, authorities monitored 21 human cases of bird flu from Egypt, Vietnam and Indonesia, including seven deaths, the organisation said.

There have also been reports of outbreaks of the virus in poultry and wild flocks in other parts of Southeast Asia, as well as in Bangladesh, Bhutan, India, Israel and Nepal, it said.

Last week, a three-year-old Vietnamese girl stricken with the virus died of severe lung infection, authorities there said.

Human cases of bird flu peaked in 2006 at 115, with 79 deaths. The number has since declined, with 73 human cases and 32 deaths in 2009, the WHO said.

However, the fatality rate for humans infected with bird flu remains high at 59 percent, it added.