The authorities ordered a halt to the sale of the tainted products which included such well-known brands as Sanlu, Mengniu, Yili and Yashili, among others.

The State Administration of Quality Supervision, Inspection and Quarantine (AQSIQ) said test samples came from 491 batches of products sold by all the 109 companies that produced the baby milk powder in the country.

The melamine content in the Sanlu brand reached 2,563 mg per kg, the highest among all the samples. In other samples, the range was from 0.09 mg to 619 mg per kilogram.

Guangdong Yashili was the only brand being exported. No melamine was found in its samples.

The chemical was not found in dairy products served for the Beijing Olympics and Paralympics.

No chemical was found in the liquid milk produced after Sept. 14.

Relevant departments have begun an investigation of the implicated companies to find out the problematic source and punish those responsible.

The AQSIQ has also sent inspectors to every dairy product factory in the country to keep a close watch over every aspect of the production process.

Dairy giant Sanlu Group apologized to the public on Monday for its contaminated milk powder that had sickened 1,253 babies with kidney stones, two fatally.

Tian Wenhua, the group's chairwoman and general manager, was fired from her posts in the wake of the scandal.

According to the Ministry of Health, the two deaths in Gansu were a five-month-old boy who died on May 1 after his family refused further treatment, and an eight-month-old girl whose family also refused an operation and removed her from hospital on July 22, the day she died.

Both were fed the Sanlu formula and suffered kidney failure.

Among the more than 1,200 sick babies, 53 had "relatively serious symptoms," the ministry said at a news conference in Beijing on Monday.

Health Vice Minister Ma Xiaowei said special hospitals had been appointed nationwide to provide treatment for all the infants with the stones.

Melamine is used in plastics and other industries and is strictly forbidden in food processing. Experts say it is added to raw milk so the protein content of the milk appears higher than it actually is.

Four people in connection with the contamination were arrested by Hebei police.

Authorities have so far seized 2,176 tons of milk powder in the warehouse of Sanlu Group. About 8,218 tons currently in the market had been recalled, said Shijiazhuang Vice Mayor Li Jinlu on Monday.

Another 700 tons was on its way back to Shijiazhuang, Li added. All the tainted milk powder would be destroyed.

Sanlu, which is 43 percent owned by New Zealand dairy company Fonterra, was ordered to halt production.