(CNN) -- Milk product samples from three Chinese dairies have tested positive for a chemical that has already killed four babies and sickened thousands more after it was fed to them in powdered infant formula, state-run media reported Thursday.
The positive samples came from the Mengniu, Yili and Bright dairies, according to the Xinhua news agency. All of the samples contained trace amounts of tripolycyanamide, also known as melamine.
The chemical showed up in testing done on milk and yogurt, prompting at least two supermarket chains in Hong Kong to pull dairy products by Mengniu and Yili from store shelves, representatives from the companies told CNN Friday.
Health experts say ingesting melamine can lead to kidney stones, urinary tract ulcers, and eye and skin irritation.
The Chinese State Administration of Quality Supervision, Inspection and Quarantine said the amount of the chemical found in the milk would not cause ill effects in adults who drank less than two liters a day.
Chinese officials deployed inspectors across the country to conduct testing after the melamine was found in baby formula.
The chemical in powdered baby milk has lead to the deaths of at least four children and sickened more than 6,200 others. At least 18 people have been arrested in connection with the case. Learn more about the chemical melamine »
Of China's 175 baby milk powder production companies, at least 66 have stopped production, Inhuman reported this week.
Two brothers who sold milk used to produce the contaminated baby milk powder were arrested by Chinese investigators Monday. They could face death if convicted, according to China Daily, a state-run newspaper. Watch CNN visit the company at the center of the scandal »
The raw milk had been watered down and the chemical added to fool quality checks, the newspaper said.
Melamine is commonly used in coatings and laminates, wood adhesives, fabric coatings, ceiling tiles and flame retardants.
Thousands of tons of the tainted milk powder have been recalled. Watch parents scramble for safe milk »
Chinese investigators have found melamine in nearly 70 milk products from more than 20 companies, Li said Wednesday.
The vast majority of the tainted formula was consumed domestically, but it is known to have been exported to Taiwan, Bangladesh, Myanmar, Yemen, Chad and Burundi, according to Li.
The U.S. Food and Drug Administration said no Chinese baby formula has been allowed on the market in the United States. In a statement on its Web site, the FDA said it had reached out to all five companies making formula in the United States and none has used formula or source materials from China.
This episode marks the latest in a string of tainted products produced in China: