MOSCOW — Russian authorities, struggling to contain an outbreak of avian flu that has killed thousands of birds in Siberia, admitted yesterday that a spread of the virus into Europe seems almost inevitable.

“It is quite likely that the flu will creep westward. What else can it do? The infection is picking up momentum,” the deputy director of the Russian Health Ministry’s Institute of Epidemiology, Viktor Maleyev, said.

Equally worrying, health officials confirmed the outbreak includes a strain that has been known to affect humans, known as H5N1. Scientists fear expansion of the virus’ geography increases the chances of a major outbreak within the human population.

While there have been isolated cases of avian flu around the world, including in America, the most dangerous strain of the virus until now has been concentrated largely in Asia, with human cases limited to Vietnam, Indonesia, Thailand, and Cambodia.

Some poultry farms on the west side of the Ural Mountains — the dividing line between European and Asian Russia — are under precautionary alerts. In Murmansk and in the populous Moscow region, chicken farms are tracking imports of birds and feed for possible signs of infection.

But authorities said the virus probably will spread along the flyways of migratory waterfowl, which within the next month or two will begin flying out of Siberia toward warmer climes along the Volga River, and eventually the Black Sea and Southern Europe. Large numbers of wild waterfowl have died on Siberian lakes, in addition to the deaths among domestic geese and chickens.

“In autumn, some wild birds migrate from the northern part of Siberia to the Caspian and Black Sea regions,” the emergency situations ministry said in a statement.

“Risks of epidemic outbreaks in the industrial poultry breeding sector therefore increases, and losses in zones of infection may be as high as 75% to 100% of the poultry population. Human infection, especially among workers at poultry farms, cannot be ruled out,” the ministry said.

Yesterday, three more districts of the Siberian Altai region reported mass deaths of poultry, and Russian television showed pictures of government workers outfitted in white biological protection suits slaughtering hundreds of other birds at infected farms.

In some towns, citizens set up roadblocks to prevent desperate farmers from evading quarantines and escaping with their flocks.

So great was the fear of human infection that health officials set up quarantine rooms in hospitals and began screening all residents of the affected areas who exhibit signs of a fever. There have been no confirmed human cases in Russia so far.

“Medical personnel will be doing the rounds from house to house,” the chief state epidemiologist, Gennady Onishchenko, said at a news conference in Novosibirsk.”What is the point of these house to house rounds? To identify people who are sick. That is to say, any temperature is enough to actively monitor that person’s health. To find out the nature of that temperature.”

While it was reported previously that a man in neighboring Kazakhstan had become infected with avian flu, it was learned this week that the man had pneumonia. Across the world, the virus has claimed 55 human lives.

Still, that remains a possibility, and a growing one, as long as the virus itself continues to expand its range, scientists say. The most pressing concern is that the virus will acquire the ability to be transmitted between humans — opening the possibility of a dangerous and widespread influenza pandemic.

Dr. Maleyev said that while the long migratory patterns of wild birds in Russia carried the possibility of transmission into Europe and, theoretically, into North America, the possible danger also is mitigated by Russia’s harsh winters, which provide a much less fertile breeding ground for disease than the warm climates of Asia.