Wal-Mart Stores has started a probe into bribery allegations in its operations outside the US after an internal investigation.

In a filing to the Securities and Exchange Commission (SEC), the world's biggest retailer said it had begun a "voluntary internal review" of its policies in relation to its global anti-corruption compliance programme. It is not known in which of the 28 countries Wal-Mart operates in the review applies to.

The filing said: "As a result of information obtained during that review and from other sources, the company has begun an internal investigation into whether certain matters, including permitting, licensing and inspections, were in compliance with the US Foreign Corrupt Practices Act."

The law makes it illegal for US citizens to bribe a foreign official. The company added that it has contacted the US Department of Justice and the SEC about its investigation, and that is implementing "remedial measures" and has brought in lawyers and other advisors to assist in the review.

It also added that it does not foresee it having a "a material adverse effect on our business, financial condition, results of operations or cash flows".

Wal-Mart has a "Statement of Ethics" on its website, which states: "We do not tolerate, permit, or engage in bribery, corruption, or unethical practices of any kind. Bribery of public officials in the US and abroad is illegal under both US law and the local law of the countries in which we operate."