However, the flu is not the same strain that has killed at least 22 people in Asia, said Dr. Ron DeHaven of the U.S. Department of Agriculture.
The strain, found in Gonzales County in South Texas, poses little threat to people, said Dr. Nancy Cox of the federal Centers for Disease Control and Prevention.
"Nonetheless, as we move forward in this particular situation, we have to keep an open mind," Cox said.
On Friday, state officials said the outbreak was a low-pathogenic version of bird flu, meaning it posed little risk to humans and only low risk to chickens.
However, it was reclassified as high-pathogenic after genetic testing during the weekend, DeHaven said.
It is the first time since 1983-84 that high-pathogenic avian flu has been found in the United States, DeHaven said.
The name and location of the chicken farm wasn't revealed.
The farm's flock was destroyed during the weekend but birds from that flock already had been sent to two live bird markets in Houston, DeHaven said.
Preliminary testing showed that the disease was found in the live-bird markets, DeHaven said. Chickens at one of those markets also were destroyed, and the same process had been started at the other market, he said.
Two other strains of bird flu have turned up in recent weeks in the eastern United States. One was detected at a farm in central Pennsylvania, and the other was found at two farms in Delaware and at live bird markets in New Jersey. None of those strains resembles the H5N1 virus that has jumped to humans in Asia.