WASHINGTON — The Agriculture Department announced Monday that an outbreak of avian flu had been confirmed in what could potentially be millions of chickens at an Iowa egg producer, the largest outbreak yet in an epidemic that has also hit turkey farmers in Minnesota.

Officials said the outbreak was at a farm in Osceola County in northwest Iowa that has about 25 barns with total of 5.3 million birds. Officials at the Agriculture Department said birds in three of the barns had been killed to stop the spread of the disease.

“But so far we don’t know how many barns are affected,” said Cullen Schwarz, a spokesman for the department, in an email.

The farm has nearly 10 percent of the state’s egg-laying hens. Iowa, the nation’s largest egg producer, has about 59 million hens that lay nearly 20 percent of the eggs consumed in the nation.