WHO chief says H1N1 virus is stable

* WHO says no sign H1N1 virus mixing with other strains

* WHO chief warns influenza has great potential for mutation

(Recasts, adds quotes, details)

MOSCOW, June 25 (Reuters) - The World Health Organization said on Thursday that the H1N1 virus was stable and there was no sign of it mixing with avian flu or other influenza viruses.

"The virus is still very stable," WHO Director-General Margaret Chan told reporters at a news briefing in Moscow when asked if there were any signs of the virus mixing with other strains such as avian flu.

"But as we all know the influenza virus is highly unpredictable and has great potential for mutation," she said.

Some health officials have said they are concerned that there is a risk that H1N1, which is commonly referred to as swine flu, could combine with the much deadlier H5N1 bird flu virus.

Chan said the viruses needed to be closely monitored to make sure they were not mutating.

"We would need to look at how they are behaving in southern hemisphere countries to see whether the H1N1 and the usual seasonal influenza virus would reassort. So far we have not detected any signal," she said.

"Another important thing we need to monitor is H1N1 and H5N1, which is endemic in some countries in Asia and the Middle East. We would like to see whether there will be any change," she said.

"Again, we did not detect any signal that there is any reassortment." (Reporting by Guy Faulconbridge, editing by Michael Roddy)