US immigration authorities have detained some 680 undocumented immigrants in what a federal prosecutor described as a record-setting operation.

US Attorney for the Southern District of Mississippi Mike Hurst told reporters the arrests took place at seven sites in six different cities in Mississippi on Wednesday. The raids, he said, are "believed to be the largest single-state immigration enforcement operation in our nation's history."

Officials declined to provide details about what sites had been targeted, citing what they said was an ongoing criminal investigation. CNN affiliates reported the raids at food-processing plants throughout the state.