Customers either don't know or aren't concerned about the practice, said Wang Jianhui, manager of the Kaiyuan Protein Feed company in the northern city of Shijiazhuang.

"We've been running the melamine feed business for about 15 years and receiving positive responses from our customers," Wang told The Associated Press in a telephone interview.

"Using the proper quantity of melamine will not harm the animals. Our products are very safe, for sure," Wang said.

Although apparently widely practiced in China, melamine use sparked concern only in March, after the U.S. recall of nearly 100 brands of pet food made with wheat gluten contaminated by the chemical. Adding melamine to food is illegal under American law, and China's government last week said it was banning its use in food products.

Melamine has no nutritional value but because it is nitrogen rich, it raises the nitrogen level of feed, making it appear that the feed is higher in protein without increasing its nutritional value. That makes it attractive to makers of feed for stock animals such as pigs, chickens and fish, as well as companies that make prepared foods for household pets such as cats and dogs.

Despite Wang's claim of safety, pet food tainted with melamine apparently has resulted in kidney failure in an unknown number of cats and other animals across the United States.

Some pet food was also shipped to hog farms in several states for use as feed and was later discovered to have another ingredient, rice protein concentrate, imported from China that was also tainted with melamine. Thousands of U.S. hogs that were fed contaminated feed were being destroyed to prevent adulterated meat from reaching consumers.

The U.S. Food and Drug Administration has blocked wheat and rice gluten from Xuzhou Anying Biologic Technology Development Co. and Binzhou Futian Bio-technology Co. after melamine was found in samples taken from batches used to make pet food.

China's government has said it will allow officials from the U.S. Food and Drug Administration to investigate melamine contamination.

Melamine is not considered a human health concern, but there is no scientific data on the health effects of melamine combined with the other compounds. Made from coal, the chemical is usually used in the making of plastics or fertilizer.

"A lot of animal food companies buy melamine from us to add in the animal feed," said Ji Denghui, manager of Sanming Dinghui Chemical Trading Co. based in the eastern province of Fujian. "This can lower the production cost and increase nitrogen levels."

Xuzhou Anying was found to have posted an advertisement on the Web site of online market place 999ce.com in March seeking to buy melamine. Other online searches for companies seeking to buy melamine were linked to companies in the chemical industry.

Xuzhou Anying's ad did not say what it intended to use the melamine for and managers have said they don't know how melamine came to be in the contaminated wheat gluten, which it claims to have purchased from another supplier.

However, Ji of Dinghui Chemical said the practice was not considered illegal and downplayed the risk.

"As far as I know, there are no rules of regulations that make this illegal. As to whether melamine is toxic or not, I believe it won't do any harm if there is only very small amount," Ji said. "Otherwise, those companies could not do that."

Calls to China's food safety regulator, the State Administration of Quality Supervision, Inspection & Quarantine, rang unanswered Monday afternoon.