WASHINGTON (Reuters) - U.S. health regulators warned Hain Celestial Group Inc about a potentially unsafe herb in some of its herbal teas, saying it might be dangerous to blood sugar, reproductive, cardiovascular and renal systems.

The U.S. Food and Drug Administration sent a letter to Hain dated August 17 calling the herb, a natural sweetener made from a South American herb called stevia, "an unsafe food additive." The agency released the letter on its Web site on Tuesday.

Stevia is being eyed by big beverage makers looking for new low-calorie sweeteners. In May, Coca-Cola Co and Cargill Inc said they would work together to market the new sweetener, despite lack of FDA approval. Stevia has been approved in a dozen other countries including Japan, China and Brazil.

The FDA letter said that although it has received requests to use stevia in food, "data and information necessary to support the safe use have been lacking."

It also said "literature reports have raised safety concerns," including those "about control of blood sugar, and the effects on the reproductive, cardiovascular and renal systems."

A spokeswoman for Boulder, Colorado-based Hain had no immediate comment.

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