U.S. Food and Drug Administration officials say they suspect the substance, which is a chemical found in plastics and pesticides, is to blame for dozens of pet deaths in North America.
China's Foreign Ministry said in a statement Thursday there was no evidence to support the FDA's claim but that it would cooperate with the United States to find out what actually killed the animals.
The ministry's comments were the first detailed response from Beijing to concerns that emerged a month ago about the country's wheat and rice gluten exports.
Meanwhile, officials in North Carolina say melamine has been found in all the hogs tested at a farm in western North Carolina. The farm has been quarantined.
The FDA said it was unclear whether any people had consumed the contaminated pork, CBS News correspondent Nancy Cordes reported.
China has said it was investigating the issue but had not acknowledged until Thursday that Chinese companies had shipped gluten tainted with melamine to the United States.
The ministry said the contaminated vegetable protein managed to get past customs without inspection because it had not been declared for use in pet food.
"There is no clear evidence showing that melamine is the direct cause of the poisoning or death of the pets," the statement said. "China is willing to strengthen cooperation with the U.S. ... to find out the real cause leading to the pet deaths in order to protect the health of the pets of the two countries."
U.S. Food and Drug Administration officials have said they suspect Chinese wheat and rice gluten laced with melamine and added to pet food may have killed at least 15 cats and dogs. The chemical appears to have caused acute kidney failure in animals that have died or been sickened after eating foods contaminated with the chemical.
China said an investigation triggered by U.S. FDA complaints found melamine in wheat and rice gluten exported to the United States by two Chinese companies: Xuzhou Anying Bio-technology Development Company and Binzhou Futian Bio-technology Company.
The case has prompted China to step up inspections of plant-based proteins and to list melamine as a banned substance for food exports and domestic sales, it said.
China also invited U.S. FDA officials to visit China to help with further investigations into the case and to consult on improving inspection techniques, it said.