WASHINGTON (AFP) -- Federal agents on Thursday arrested hundreds of illegal immigrants working at Wal-Mart stores in 20 states in a crackdown on illegal employment of aliens, the Department of Homeland Security (DHS) announced.

"These arrests are part of ICE's (Immigration and Customs Enforcement) ongoing efforts to ensure that U.S. companies do not employ individuals who are unauthorized to work in the United States," the DHS said in a statement. "Individuals who have violated U.S. immigration laws by illegally entering and working in the United States are subject to arrest and removal from this country," it said. "Those arrested today will be put into removal proceedings by ICE and appear before a U.S. immigration judge."

Wal-Mart, the world's largest retailer, said in a statement late Thursday that more than 300 workers in 61 of its stores were arrested, and that it was cooperating with the investigation.

It did not comment on a search of its corporate headquarters in Bentonville, Arkansas.

"We are currently trying to understand the scope and details of the investigation," said spokeswoman Karen Burk.

The coordinated swoop by ICE officers, the third involving Wal-Mart stores in five years after ones in 1998 and 2001, was conducted as workers were coming off their night shifts.

Wal-Mart said those arrested were members of night cleaning crews the company hired through private contractors.

An ICE source said the arrests were made after federal officials cross-checked the employees' documentation, and that the workers were mainly Eastern European nationals.

The Wal-Mart spokeswoman said the company relies on more than 100 contractors to furnish workers to clean its more than 700 stores in the United States.

"We require each of these contractors to use only legal workers ... We are talking to (the ICE) and are committed to cooperating with them," she said.