Traces of the industrial chemical melamine have been detected in samples of top-selling U.S. infant formula, but federal regulators insist the products are safe.

The Food and Drug Administration said last month it was unable to identify any melamine exposure level as safe for infants, but a top official said it would be a "dangerous overreaction" for parents to stop feeding infant formula to babies who depend on it.

"The levels that we are detecting are extremely low," said Dr. Stephen Sundlof, director of the FDA's Center for Food Safety and Applied Nutrition. "They should not be changing the diet. If they've been feeding a particular product, they should continue to feed that product. That's in the best interest of the baby."

Melamine is the chemical found in Chinese infant formula — in far larger concentrations — that has been blamed for killing at least three babies and making at least 50,000 others ill.

Previously undisclosed tests, obtained by The Associated Press under the Freedom of Information Act, show that the FDA has detected melamine in a sample of one popular formula and the presence of cyanuric acid, a chemical relative of melamine, in the formula of a second manufacturer.

Separately, a third major formula maker told AP that in-house tests had detected trace levels of melamine in its infant formula.

The three firms — Abbott Laboratories, Nestle and Mead Johnson — manufacture more than 90 percent of all infant formula produced in the United States.

The FDA and other experts said the melamine contamination in U.S.-made formula had occurred during the manufacturing process, rather than intentionally.