MOSCOW--The swine flu virus is "stable" but must be monitored closely, director general of the World Health Organisation Margaret Chan said Thursday, Russian news agency reports said.

"The virus is not mutating for the moment, it is stable," said Chan after a meeting with Russian Health and Social Development Minister Tatiana Golikova.

Chan, however, underlined the importance of close monitoring of the spread of the virus, adding that it was highly "unpredictable."

Since it emerged at the end of April, experts have feared that it could mutate into a virus that combines the contagious nature of swine flu with the virulence of avian flu.

The WHO declared a global pandemic earlier this month. In a global update on Wednesday it reported 55,867 laboratory-confirmed cases of the A(H1N1) virus in 109 countries and 238 human deaths from the disease since late March.