Minnesota, the top U.S. turkey producing state, has found a fourth turkey flock to be infected with a virulent strain of avian flu, the U.S. Department of Agriculture said on Thursday. 

South Dakota has found its first infection of the H5N2 flu strain, which can kill a poultry flock within 48 hours, in turkeys, according to the agency.

Recent infections of avian flu in states stretching from Arkansas to Oregon have prompted overseas buyers to limit imports of U.S. poultry from companies such as Tyson Foods Inc, Pilgrim's Pride Corp and Sanderson Farms Inc.

No human infections of the virus have been detected, USDA said.