Samples of milk powder found in northwest China's Gansu and Qinghai provinces had levels of the chemical melamine up to 500 times beyond the permitted limit, and suspected tainted powder has also turned up in the country's northeast, said the report from the Xinhua news agency on Friday.

The exposure of tainted milk products in a poor and remote parts of China's northwest has underscored the persistence of food safety problems that have alarmed consumers and sparked criminal scandals that led to executions and official sackings.

Two years ago, at least six children died and nearly 300,000 children fell ill from drinking powdered milk laced with melamine, an industrial compound added to fool inspectors by giving misleadingly high results in protein tests.

Melamine can cause kidney stones, and is used to make plastics, fertilizers and concrete. Its high nitrogen content allows protein levels to appear higher when added to milk or animal feed, allowing traders to disguise substandard products.