The government has cancelled the business licenses of two firms that exported wheat protein tainted with toxic chemicals that wound up in pet food in the United States, a senior quality control official said Friday.

U.S. consumers have been alarmed by a spate of pet deaths blamed on tainted wheat gluten and rice protein exported from China, as well as reports of toxins and disease in other Chinese exports.

The two companies -Xuzhou Anying Biotechnology Development in Jiangsu Province and Bingzhou Futian Biotechnology in Shandong Province - "unlawfully added melamine in some of their protein products exported to the United States,"quality watchdog chief Li Changjiang told a news conference.

He was speaking a day after China warned the United States against "groundless smear attacks" against Chinese products and said it was working responsibly to address concerns over a spate of recent food safety scares.

"I think that the quality of products from the great majority of food manufacturers is guaranteed. At the same time, we will enhance the rectification and supervision over products produced by small food manufacturers whose market share is only 10 percent," Li said Friday.

"We will resolutely close down these small food manufacturers or workshops who are engaged over the years in producing sub-standard, or fake and shoddy food."

The deaths of patients in Panama from mislabeled drug ingredients from China as well as a series of other cases have raised fears about the safety of China's surging exports.

On Friday, the State Council, or cabinet, said the firm behind the Panama poisonings had also had its business license revoked.

But Li defended a company that made tires the United States has recalled for allegedly being sub-standard, a firm that produced ceramic cooking pots Japan claimed contained lead, and a candy maker whose product has been banned in the Philippines for purportedly containing formaldehyde and other harmful substances.

"The Chinese Government has not turned a blind eye or tried to cover up. We have taken this matter very seriously, acted responsibly and immediately adopted forceful measures," said a statement by China¡¯s embassy in Washington on Thursday, warning the United States against "groundless smear attacks" against Chinese products.

"Blowing up, complicating or politicizing a problem are irresponsible actions and do not help in its solution,¡± the Chinese mission said in a rare policy pronouncement.