WASHINGTON — A strain of H5N1 bird-flu virus found in an infected Vietnamese girl is resistant to the drug being stockpiled by more than a dozen countries, including the United States, as a defense against a possible global pandemic, researchers reported yesterday.

The new finding, while not unexpected, raises the possibility that oseltamivir, sold as Tamiflu, might be less useful than anticipated if resistant-strains of the H5N1 avian-flu virus become more prevalent and the virus gains the ability to pass easily from person to person — a trait it does not possess now.

Also yesterday, after confirmation that the H5N1 bird-flu virus had reached Turkey and may be in Romania, the European Union (EU) announced new measures to combat its spread, telling governments to pinpoint the areas most at risk and to separate poultry from wild birds, which carry the virus.

News services reported that the commission acted on the advice of health and veterinary experts who held an emergency meeting after learning that avian flu had reached Europe's doorstep.

Yesterday's report is the first indication that tests have detected a drug-resistant strain of H5N1 since the virus began circulating among birds in Asia.

It was found in a 14-year-old Vietnamese girl who became ill in February while caring for her brother, who also was infected. She had received a low preventive dose of Tamiflu, and then a higher dose when she became ill. She recovered fully.

The report, by Q. Mai Le of Vietnam's National Institute of Hygiene and Epidemiology, in Hanoi, and 15 international collaborators, will appear in next week's issue of Nature.

Tamiflu is in a class of antiviral drugs called neuraminidase inhibitors. Although resistant to Tamiflu, the strain reported yesterday remained susceptible to another member of the class, zanamivir, sold as Relenza. All H5N1 viruses are resistant to the other main class of flu drugs, adamantanes.

The H5N1 strain has infected 117 people in Southeast Asia and killed 60 since December 2003. It also has killed, or led to the culling of, 140 million domestic birds.

Genetic fingerprinting of the resistant strain suggests that the girl was infected by her brother, as their viruses were extremely similar. But it appears that the resistance developed in the girl.

The U.S. Strategic National Stockpile of drugs contains 2.3 million treatment courses of Tamiflu, with more on order, but only 83,000 courses of zanamivir (Relenza).