By Maggie Fox, Health and Science Editor

WASHINGTON (Reuters) - Seasonal flu viruses are developing the ability to evade influenza drugs globally, but how and why this is happening is not clear, experts told a conference on Monday.

Europe is the worst-affected by strains of influenza that resist the effects of antiviral drugs, but the resistance is growing globally, they told a meeting of the Infectious Diseases Society of America.

"A significant proportion of resistant viruses were observed in Europe this winter," Dr. Bruno Lina of Claude Bernard University in Lyons, France, told the meeting.

The resistance also varies by strain, with a quarter of H1N1 flu viruses resistant in Europe and about 11 percent of H1N1 in the United States, but far fewer cases of H3N2 and influenza B viruses.

Their findings show that flu viruses -- already known to mutate speedily -- may be even more unpredictable than anyone thought.

Experts fear drugs may become quickly useless to fight an unusually severe flu season or the emergence of a new strain of flu that may cause a pandemic. They have been stressing the need to develop new flu drugs and also quicker and better ways to make vaccines.

The World Health Organization and the U.S. Centers for Disease Control and Prevention have been collecting samples of the annual flu viruses to check them against the four available flu drugs: amantadine and rimantadine, and the newer drugs Tamiflu and Relenza.

The viruses changed rapidly over the past 2007-2008 flu season, Lina said. "We started with something like 10 percent in Europe," Lina said. By April of this year, 25 percent of the viruses tested were resistant to Tamiflu.