* FDA says L'Oreal claiming creams work like drugs

* Companies that sell drugs must do clinical trials

WASHINGTON, Sept 11 (Reuters) - U.S. regulators warned L'Oreal SA, the world's biggest cosmetics group, to stop advertising skincare products using language that makes them sound like drugs.

The U.S. Food and Drug Administration said Lancome USA, a L'Oreal unit, claimed some of its skin creams could "boost the activity of genes" or "stimulate cell regeneration" to reduce signs of aging.

Any product that is intended to affect the structure or function of the human body is classified as a drug, the FDA said, according to a warning letter posted on its website on Tuesday.

Companies are not allowed to sell drugs in the United States without submitting an application to the FDA proving their products are safe and effective and then winning FDA approval.

L'Oreal, a French company, was not immediately available for comment.

Some of the products mentioned in the letter, dated Sept. 7, include Genifique Repair Youth Activating Night Cream and Absolue Eye Precious Cells Advanced Regenerating and Reconstructing Eye Cream.

On its website, Lancome describes the Genifique Repair cream, which costs $98 for a 1.7-ounce container, as "our first night care that boosts the activity of genes." The company cites an in-vitro test on genes to back up the claim.

The FDA said failure to fix the advertising claims could lead to enforcement actions, such as seizure of the products and injunctions against their manufacturers and distributors.