Federal agents have raided the headquarters and 60 stores of US retail titan Wal-Mart, arresting more than 300 illegal immigrant workers.

The workers were part of cleaning crews hired by outside contractors, but Wal-Mart may have known about the violations, sources told the Associated Press.

"We have seen no evidence of this from the INS, and, if that turns out to be true, we will cooperate fully with law enforcement officials," Wal-Mart spokeswoman Mona Williams was quoted by AP as saying.

The arrests were made at Wal-Mart stores in 21 US states, while boxes of documents were taken away from an executive's office at Wal-Mart's Bentonville headquarters.

Law enforcements sources told the Associated Press that the investigation had grown out of earlier investigations into Wal-Mart cleaning crew contractors in 1998 and 2001.