Liquid milk sold by three leading companies is contaminated with melamine, tests showed yesterday, and the number of infant deaths caused by the chemical rose to four.

The fourth death was reported from the Xinjiang Uygur autonomous region. Two babies in Gansu and one in Zhejiang had fallen victim to contaminated milk earlier. A total of 6,244 infants who were fed contaminated milk food produced by Sanlu Group have been diagnosed with various urinary tract problems, including kidney stones.

Police in Shijiazhuang, where Sanlu is based, said 12 people were arrested yesterday for their alleged involvement in the milk food scandal, taking the total number of arrests to 18.

The State Council has decided to do away with the system of exempting certain food products from inspection. The government began the exemption system in 2000 to help companies that produced high-quality, globally competitive products avoid the hassle of facing repeated tests.

Tests conducted on liquid milk over the past few days showed 24 of the 1,202 batches were contaminated, the General Administration of Quality Supervision, Inspection and Quarantine (AQSIQ) said in a report.

The AQSIQ, however, quoted scientists as saying the contamination level in liquid milk did not pose a big threat to people's health. Even milk with the highest concentration of melamine is safe for a 60-kg or heavier adult if he or she drinks up to 2 liters a day.

The AQSIQ yesterday said the tainted products came from the Yili and Mengniu groups, both based in the Inner Mongolia autonomous region, and the Shanghai-based Bright Dairy.

It has asked the three companies to recall all their contaminated products.

Eleven of the 121 Mengniu batches that were tested had melamine, a chemical that makes milk appear rich in protein during quality tests, but can also cause kidney problems. Melamine is used to make plates, bowls, mugs and other sundry products but is banned from being used in the food industry.

The highest concentration of melamine was found in Bright Dairy products. Every kg of its liquid milk contained 8.6 mg of melamine.

The three firms' milk powder products, too, had tested positive for melamine, and they had started recalling those products.

The fourth melamine victim died in a hospital in Bayingolin, the Xinjiang regional health department said on its website.

The scandal has claimed the heads of five government officials of Shijiazhuang, capital of Hebei province, and Sanlu board chairman Tian Wenhua. The highest-ranking official to fall is the mayor of Shijiazhuang Ji Chuntang, who resigned late on Wednesday.

Shi Guizhong, spokesman for the Hebei public security bureau, said six of the arrested people are suspected to have sold melamine and the rest, all milk suppliers, of mixing the chemical with milk before selling it to Sanlu.

One suspect surnamed Su admitted to have sold 4 tons of melamine to local dairy farms and milk collection centers between February 2007 and July 2008. Su said he used to buy a 20-kg bag of melamine for 200 yuan ($28.6), and resell it for 218 yuan.