HOHHOT, Oct. 23 (Xinhua) -- Police in north China have arrested six men on suspicion of adding fake protein formula to fresh milk supplied to Mengniu, one of the country's biggest dairy companies.

The men, aged from 37 to 57, were all natives of Inner Mongolia Autonomous Region, said a spokesman with the public security bureau in the regional capital Hohhot.

Three were suspected of having mixed fake protein with water and substandard whey powder into the fresh milk they collected from dairy farms across the region. The milk was later found to contain melamine.

The other three suspects were accused of providing low-quality melamine-tainted protein to the milk dealers, the spokesman said.

One milk dealer, surnamed Cui, allegedly confessed to adding 100 kilograms of water and 600 grams of fake protein to the 2,100 kilograms of fresh milk he delivered to Mengniu on Sept. 18.

In a dairy quality overhaul following the melamine-tainted milk scandal that hospitalized thousands of babies with kidney stones across China, police seized 19 kilograms of fake protein in Cui's house. Tests found an average 307 grams of melamine in every kilogram of the white powder.

Sun, a fake protein dealer who sold Cui 20 kilograms of the substandard "protein" for 380 yuan (54.3 U.S. dollars), was also arrested. Police said the product on sale at his store in Hohhot had no description of its ingredients, date of production or name of manufacturer.

Two other dealers were arrested for allegedly selling melamine-tainted whey powder to milk dealers.

Police are still investigating the cases.

In north China's Hebei Province, where the melamine-tainted Sanlu infant formula was made, 36 people have been arrested in connection with the scandal, including Sanlu Group's chairwoman and general manager Tian Wenhua.

As of Wednesday, 3,654 infants across China were still in hospital with kidney diseases caused by tainted powdered milk, including three seriously ill, the Ministry of Health said on its website.