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BEIJING, April 26 (Reuters) - China denied on Thursday that grain protein products its companies sold to the United States caused a spate of pet deaths, but it promised to tighten controls and ban a suspect chemical from the food ingredients.

U.S. Food and Drug Administration officials have said melamine, a chemical used in plastics and fertilizer, was found in wheat gluten and rice protein imported from China for use in some pet foods.

More than 100 brands of pet food have been recalled after reports of kidney failure in pets, and U.S. inspectors say they suspect traces of melamine.

China's Foreign Ministry issued a statement denying that the chemical was the culprit. "At present, there is no firm evidence to show that melamine was the direct cause of the poisoning and death of the pets," said the faxed statement.

But the ministry also spelled out steps it said would ensure that Chinese wheat and rice protein products would meet foreign safety standards and "avoid such cases recurring".

Exports of these products from two companies identified by U.S. officials as sources of tainted goods have been banned, inspections of the protein exports would be tightened, and melamine-tainted protein products would be banned from exports and domestic markets, the ministry said.

On Thursday, Chinese Foreign Ministry spokesman Liu Jianchao also said that U.S. Food and Drug Administration officials had been invited to China to work through the case.

"China is also concerned about this incident," Liu said. "China will actively cooperate."

Two Chinese companies at the centre of the dispute had not declared the exports to U.S. customs inspectors as petfood ingredients, the Chinese statement said. Instead, the shipments were declared as products that did not require inspection.

U.S. officials have identified the two Chinese companies to be Binzhou Futian Biology Technology Co. Ltd. and Xuzhou Anying Biologic Technology Development Company Ltd.

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