By Susan Heavey

WASHINGTON (Reuters) - The U.S. Food and Drug Administration defended the safety of infant formula sold in the United States on Friday despite tests that found the chemical melamine in one brand and a related compound in another.

FDA tests found "very low levels" of the industrial chemical melamine in Nestle's Good Start Supreme with Iron formula, said Stephen Sundlof, director of the FDA's Center for Food Safety and Applied Nutrition.

It also found low levels of cyanuric acid in Mead Johnson's Enfamil Lipil with Iron, Sundlof said. Mead Johnson is a unit of Bristol-Myers Squibb.

Those findings "do not raise public health concerns," Sundlof told a conference call. "The domestic supply of infant formula is safe."

Representatives for Nestle and Bristol-Myers could not immediately be reached for comment.

The FDA earlier this week said it had found one brand of formula containing melamine, a chemical used widely in the production of plastics and fertilizer, but declined to name the company to Reuters.

On Friday, the FDA said it had so far tested 74 samples of U.S. infant formula, a process that began in September when melamine was discovered in infant formula in China.

Thousands of Chinese children were sickened and at least four died after developing painful kidney stones from formula contaminated with melamine, which has been used as a cheap substitute to boost the appearance of protein levels in milk and other products.

The FDA has 13 more samples left to test, it said.

It is not clear how the chemicals ended up in the two samples that tested positive so far, Sundlof said.

"Melamine for certain is an approved food contact substance. It is used in packaging materials... it is used in some can liners, and there is the possibility of migrating," he said. Cyanuric acid may also be used in sanitizers, he added.

(Additional reporting by Rachelle Younglai; Editing by Tim Dobbyn)