Shijiazhuang, China ? China’s food safety crisis widened Friday after the industrial chemical melamine was found in milk produced by three of the country’s leading dairy companies – prompting stores, including Starbucks, to yank milk from their shelves.

The recalls come as evidence is mounting that adding chemicals to watered-down milk was a widespread practice in China’s dairy industry.

Sipping from a carton of milk at a news conference, the chief financial officer of one of the companies, Mengniu, apologized for the tainted milk. But he insisted only a small portion of the company’s inventory had been contaminated and said the tainted milk came from small-scale dairy farmers.

“Large-scale milk farms are very disciplined. They won’t take the risk to do something like that,” Yao Tongshan told reporters in Hong Kong.

The crisis was initially thought to have been confined to tainted milk powder, used to make baby formula that has been blamed in the deaths of four infants and for sickening 6,200 other children.

But tests found melamine in samples of liquid milk taken from China’s two largest dairy producers, Mengniu Dairy Group Co. and Yili Industrial Group Co., as well as Shanghai-based Bright Dairy. The chemical, which is used in plastics and fertilizers, can cause kidney stones and lead to kidney failure.

All batches that tested positive were being recalled, China’s product safety watchdog said in a report on its Web site. It pledged to “severely punish those who are responsible.”

Melamine, which is high in nitrogen, makes products with it appear higher in protein. Suppliers trying to cut costs are believed to have added it to watered-down milk to cover up the resulting protein deficiency.

No tainted infant formula has turned up in the United States, where authorities have inspected more than 1,000 retail markets mainly serving Asian communities. China is an importer of liquid milk, so it’s unlikely that milk from that country would have been shipped to the U.S.

But the Food and Drug Administration said it is stepping up inspections at ports as a precaution. Inspectors will be sampling bulk shipments of food ingredients from Asia that are derived from milk, such as milk powder and whey powder. The FDA also plans to issue a consumer alert warning people not to buy milk products from China on the Internet.

Starbucks Corp. said its 300 cafes in mainland China were pulling all milk supplied by Mengniu, though the Seattle-based company said no employees or customers had fallen ill from the milk.