Federal food safety officials began holding up shipments of food from China that contain milk or milk-derived ingredients in the largest effort to date to keep products tainted with the industrial chemical melamine from reaching U.S. consumers.

The Food and Drug Administration is requiring importers of the halted shipments to test for the chemical, which is used to make plastic and fertilizer but has been added to human and animal food to boost protein readings. The types of products likely to be waylaid are cookies, candies, and other goods made with milk or milk powder.

Since September, FDA officials have recalled several products -- sold mainly in ethnic grocery stores -- due to possible melamine contamination. They chose to take broader measures yesterday based on the results of product testing and on information from food safety officials in other countries.