The US watchdog said the unnamed formulas contained traces of perchlorate, a salt derivative used in fireworks and rocket fuel and to treat thyroid gland disorders.

The US Environmental Working Group said a high level of the chemical could interfere with thyroid hormone production, though the study found the levels of perchlorate in the baby formula to be safe.

Milk-based formula were said to contain more perchlorate than their soy-based counterparts, and two brands that together had 87 percent of the country's market share contained more than double the amount of perchlorate than their competitors.

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Hong Kong's Center for Food Safety said it will be following up on the matter.

A CFS spokeswoman said the center was aware of the report and will be gathering information and details. No action was deemed necessary at this time.

"Regarding the report on the chemical perchlorate found in baby formula, no brand names were disclosed," the spokeswoman said. "Also, the US authorities did not advise any warning or issue any recall."

She said CFS' current food surveillance program does not test for the presence of perchlorate in baby formula and the center could not comment on any testing following the US report.

The program samples foods at the import, wholesale and retail levels to check on microbiological and chemical safety.

Perchlorate is the latest substance found in baby formula. China is still suffering fallout from the melamine-in-the-milk shock, when contaminated baby formula was linked to the deaths of six infants and hundreds of thousands of youngsters falling sick across the country.

The scandal erupted last September and was soon felt in Hong Kong. There was a massive recall of infant formula and snack foods.