China says further checks on food exporters have turned up no sign of a chemical blamed in the pet deaths in North America, and urged U.S. authorities not to take additional measures against Chinese producers.

The government body responsible for overseeing food safety said it accompanied U.S. Food and Drug Administration inspectors on visits to two companies blamed for the chemical contamination on May 8-12 following the publication of the results of China's own investigation.

The incidents involving Xuzhou Anying Biologic Technology Development Co. Ltd. and Binzhou Futian Biology Technology Co. Ltd. were "special individual cases," the General Administration of Quality Supervision, Inspection and Quarantine said in a statement viewed on its Web site Wednesday.

U.S. inspectors said wheat gluten exported by the firms and used to make pet food was tainted with the mildly toxic melamine and caused the deaths of an unknown number of dogs and cats, sparking a recall of 154 brands of pet food contaminated with the chemical.

Chinese authorities have detained an unknown number of managers from the two companies.

The statement said FDA inspectors also expressed satisfaction with the quality controls and tracing measures in place at another exporter of vegetable protein, Sinoglory, saying those met U.S. production standards for similar products.

"China emphasizes that its determination to crackdown on law breaking enterprises is firm and its policies are effective," said the statement.

"We hope the American side will accurately and objectively deal with problems among individual companies and not take stringent measures against other Chinese companies producing the same type of products," it said.

China says the two companies added melamine to the gluten after failing to provide the protein level required in their contracts.

Melamine, used in plastics, fertilizers and flame retardants, has no nutritional value but is high in nitrogen, making products to which it is added it appear to be higher in protein.

China has also accused the companies of illegally mislabeling their exported products to avoid inspections.

U.S. officials say they don't believe melamine to be harmful to humans, but say they have too little data to determine how it reacts with other substances.