Moscow ? Russian officials have quarantined a large poultry farm in Siberia because of a suspected outbreak of bird flu, news reports said Saturday. If confirmed, it would be the first major occurrence of the lethal virus among birds in Russia, and international health officials expressed concern that the disease had spread closer to Western Europe.

About 142,000 birds are being monitored at a commercial farm in the Omsk region of Siberia, 1,400 miles east of Moscow, the Russian news agency Interfax reported, quoting a federal agency that tracks the disease. The presence of the deadly H5N1 strain of avian influenza was reported last month in Siberia, but only among wild birds and free-range chickens on small family farms.

Avian influenza has killed at least 61 people in Vietnam, Thailand and Cambodia since early last year, mostly farmers and poultry workers in close contact with the animals. Millions of birds have been slaughtered in Asia in an attempt to control the disease.

The World Health Organization has warned that the viral strain affecting chickens, ducks and wild fowl could develop into a form that spreads easily among humans, exposing millions of people to the disease. The exact means of transmission is unclear, and it is not conclusively known whether the disease can be passed by eating infected poultry.