MADRID/GENEVA (Reuters) – The World Health Organization is likely to raise its flu alert to the top of its six-point scale and declare a pandemic, its director-general indicated in an interview published on Monday.

In remarks setting the scene for another alert increase, but without saying when, WHO chief Margaret Chan warned against over-confidence following a stabilization in the number of new cases of the H1N1 strain that has proved deadly in Mexico.

"Level 6 does not mean, in any way, that we are facing the end of the world. It is important to make this clear because (otherwise) when we announce level 6 it will cause an unnecessary panic," she told Spanish newspaper El Pais.

"Flu viruses are very unpredictable, very deceptive ... We should not be over-confident. One must not give H1N1 the opportunity to mix with other viruses. That is why we are on alert."

The WHO's pandemic phases reflect views about how a virus is spreading, and not how severe its effects are.

Last week the United Nations agency raised the alert level twice, from 3 to the current 5, in response to the sustained transmission of H1N1 in Mexico and the United States.

Before issuing a level 6 alert, the WHO would need to see the virus spreading within communities in Europe or Asia.

A declaration of a full pandemic would send a signal to governments worldwide to institute their pandemic response plans, which may include measures affecting hospitals, schools or public events.