"Analysis showed that the swans died from the H5 bird flu infection," the emergency situations ministry was quoted as saying by the official RIA Novosti agency.
Russia's deputy veterinary chief, Yevgeny Nepoklonov, told state-run
ITAR-TASS that roughly 250 swans had died and that a quarantine was being enforced around the area near the city of Astrakhan.
Officials did not say whether it was suspected the flu could by the H5N1 variety, a deadly strain of which has killed more than 60 people in Asia in the past two years.
H5N1 has so far been detected in birds in eight Russian provinces and hundreds of thousands of birds have been slaughtered in an effort to quell the virus, which officials say was brought from Asia by migratory birds.
Experts fear a pandemic that could kill millions of people across the globe if H5N1 acquires genetic material from a human influenza virus and becomes easily transmittable from human to human.