NEW YORK (CNN/Money) - Wal-Mart Stores Inc. said Thursday it will begin criminal background checks on job candidates for its Wal-Mart stores and Sam's Club membership warehouses in the United States.

The largest U.S. private employer announced the policy Thursday after it reported second-quarter results. It said the checks will begin in the Midwest in September and will start across the country in the following months.

"By adding another level of security to our hiring practices, our associates can be assured that we are strengthening our efforts to try to intercept anyone who might otherwise damage that integrity," said Wal-Mart Vice President Sue Olliver in a statement.

Wal-Mart (WMT: up $1.21 to $52.84, Research, Estimates), which employs 1.5 million people worldwide, has been criticized for two incidents in South Carolina where employees with prior criminal convictions for sexually-based offenses were accused of sexually assaulting young girls, according to the Wall Street Journal.

In one of the incidents, the employee was convicted, and charges are pending in the other, the paper reported.

Wal-Mart plans the criminal background checks only on people deemed qualified candidates for actual hiring. It already conducts such checks on some personnel, including loss-prevention and pharmacy employees, according to the paper.

Wal-Mart faces other legal problems. It has denied knowingly hiring contractors who used illegal immigrants to clean its stores, and has reportedly been in talks to settle a Department of Justice probe into the matter.

Meanwhile, a federal judge this year certified a class-action workplace bias suit against Wal-Mart, covering as many as 1.6 million current and former female employees.  

-- from staff and wire reports