China arrested six people Thursday for their alleged role in supplying contaminated milk to dairy companies, as the Health Ministry said more than 3,600 Chinese children remain hospitalized after consuming compromised products.

The six suspects, who worked in the major dairy-producing region of Inner Mongolia, allegedly either sold melamine to milk suppliers or added the industrial chemical to milk themselves, the official Xinhua News Agency said.

Melamine, used in plastics and fertilizer, can cause kidney stones and lead to kidney failure in larger doses. A total of 3,654 children remain sick, with three in serious condition, the Health Ministry said in a notice on its website late Wednesday.

Authorities say middlemen apparently added melamine to milk they collected from farmers to sell to large dairy companies. The suppliers are accused of watering down the milk and then adding the nitrogen-rich chemical to make the milk seem higher in protein when tested. Protein tests often simply measure nitrogen levels.

As of Wednesday, a total of 46,717 children had been treated and discharged from hospitals, the ministry said. Milk powder contaminated with melamine has been blamed for the deaths of four infants.

There have not been any more reports of deaths, the ministry said, adding that all the deaths occurred between May and August, before the public knew milk products were tainted.

UN report calls for stricter surveillance

Xinhua said the government in Hohhot, Inner Mongolia's capital, requested that police launch an investigation into the sources of milk for China's largest dairy companies, Yili Industrial Group and Mengniu Dairy Group Co. — both of which are headquartered in the region.

Three of the suspects, who operated milk collecting stations, allegedly put additives containing melamine bought from two other suspects into their milk so it could pass quality testing, Xinhua said. The report said the sixth suspect is accused of selling a range of manmade additives containing melamine but did not give any other details. The milk collection stations sold milk to Mengniu, it said.

The government and the dairy companies have blamed the contamination of their milk products on the system of scattered milk collection stations across the grasslands of which they say a few criminal groups took advantage.

Since authorities announced melamine was found in a host of milk products in September, the scandal has prompted a string of recalls of Chinese-made milk and products containing milk in dozens of countries. Yili and Mengniu have seen their share prices plummet.

The companies say they have bought new testing labs for melamine, and are trying to consolidate farms that supply their milk in an effort to win back consumer confidence.

A United Nations report Wednesday urged the country to enact stricter laws and replace its patchwork surveillance system to help restore public trust badly shaken by a spate of food safety scandals.

Most critically, China needs a unified regulatory agency, the report said, and a place consumers can go for reliable information. The task is now split among a half-dozen government agencies, creating confusion and uneven enforcement, it said.