But consumer advocates said the agency’s action was too little and too late.

“Although F.D.A.’s action today is a step in the right direction, it does not do enough to ensure consumer safety, especially since melamine contamination in Chinese products continues to broaden,” said Wenonah Hauter, executive director of Food & Water Watch.

As a result of the latest alert, Chinese products that contain milk or milk powder will automatically be detained at the border until the manufacturer or its customer has the product tested and it is found to be free of contamination, or they show documentation indicating that the product does not contain milk or milk-derived ingredients.

“The burden shifts to the importer,” Dr. Solomon said.

F.D.A. analyses have detected melamine and cyanuric acid, another contaminant, in “a number of products that contain milk or milk-derived ingredients, including candy and beverages,” according to an alert that the agency sent to field personnel. The alert also noted that inspectors in more than 13 other countries had discovered melamine in Chinese products including milk, yogurt, frozen desserts, biscuits, chocolates and cookies.

The agency routinely blocks imports of individual food products, but it is rare for it to block an entire category of one country’s foods. Last year, the F.D.A. blocked five types of farm-raised seafood as well as vegetable protein from China because of repeated instances of contamination.

Unscrupulous food and feed dealers in China add melamine to their products because it increases nitrogen content to give the appearance in testing that protein levels are adequate. Because it dissolves poorly, melamine can block the body’s filtering system, potentially leading to kidney failure and death.