More than 300 illegal workers have been seized in raids at 61 Wal-Mart stores in 21 states across the US as part of an immigration crackdown by federal agents.

The workers were members of cleaning crews hired by outside contractors, and were arrested as they were coming off their night shifts. They are believed to be mainly Eastern European nationals.

The Department of Homeland Security (DHS) said in a statement that: "These arrests are part of ICE's (Immigration and Customs Enforcement) ongoing efforts to ensure that US companies do not employ individuals who are unauthorised to work in the United States.

"Individuals who have violated US immigration laws by illegally entering and working in the United States are subject to arrest and removal from this country.

"Those arrested today will be put into removal proceedings by ICE and appear before a US immigration judge."

Wal-Mart Stores, which last year had sales of $244.5 billion, uses more than 100 third-party contractors to clean more than 700 stores nationwide.