China acknowledged on Tuesday that two Chinese companies had illegally exported contaminated wheat gluten and rice protein for pet food blamed for a spate of animal deaths in the United States.

U.S. Food and Drug Administration (FDA) says it has has received about 17,000 complaints of sick pets, with about 4,000 deaths reported. Investigations have extended to livestock feed containing tainted pet food that made its way to some 6,000 hogs and as many as 3.1 million chickens.

The Agency said there was little danger for people.

China had denied FDA assertions that melamine, a chemical used in plastics and fertiliser, had been added to wheat gluten and rice protein exported from China for pet foods. But Xinhua state news agency reversed the official position.

"The two companies illegally added a bid to meet the contractual demand for the amount of protein in the products," Xinhua said, quoting the General Administration of Quality Supervision, Inspection and Quarantine.

The Administration said the two companies, named by the FDA as Xuzhou Anying Biologic Technology Development Co. Ltd. and Binzhou Futian Biology Technology Co. Ltd, evaded quality checks by labeling their products as exports not subject to inspection.

Local police had launched an investigation into the case and detained company officials.

The quality control watchdog said it had ordered local branches to strengthen quality inspection on all vegetable proteins and pledged to include all vegetable proteins in the exports list subject to quality checks.

The watchdog said it had notified the FDA of the investigation results and proposed to set up a cooperation mechanism on food safety with the United States.