China's dairy industry is on the verge of collapse
Chinese police have arrested 22 people suspected of producing melamine - the chemical found in milk products which have made thousands of babies ill.
The arrests took place on 17 September, but were only reported widely on Monday evening.
Police are said to have raided dairy farms and milk purchasing stations in northern Hebei province, and seized more than 220kg (485lb) of melamine.
Meanwhile more Chinese milk products have been recalled.
In the latest recalls:
- Anglo-Dutch giant Unilever announced it was withdrawing Lipton-brand milk tea powder in Hong Kong and Macau, after internal tests found traces of the chemical in four batches of the product
- Hong Kong authorities said they found melamine in three other products: Pocky Men's Coffee Cream Coated Biscuit Sticks (made by Japan's Ezaki Glico Co) and Walnut Cakes and Coconut Cakes (made by China's Tian Le Yuan Foods)
As well as making about 50,000 babies ill, the tainted milk products have also been blamed for the deaths of four babies.
The Chinese news agency Xinhua said that of the 22 people detained, 19 were managers of 17 pastures, breeding farms and milk purchasing stations.
Later reports suggested another five arrests had been made, though where and when they took place was unclear.
More than 800 police were involved in the raids on 41 locations in Shijiazhuang, the capital of Hebei, Xinhua said.
Police said the melamine had been produced in underground plants and then sold to breeding farms and purchasing stations.
Xinhua quoted one of those arrested as saying he had been producing "protein powder" containing melamine since last year.
Another suspect was quoted as saying his milk had always been rejected by Sanlu, the company where the toxic milk was first discovered last month which is headquartered in Shijiazhuang.
He said he was told that lacing his milk with melamine could increase the proportion of protein and would help his milk pass the Sanlu test. So he began tainting the milk, Xinhua reported.
Chinese President Hu Jintao was shown on state TV news on Tuesday night touring a dairy farm in the eastern province of Anhui.
In some of his most public comments on the scandal so far, he was shown telling an official at the farm:
"Food safety concerns the health of the public.
"We need to ensure that all products on the market are up to standard, so that consumers don't have to worry."
Thousands of children have been hospitalised with kidney illnesses and four have died after drinking milk formula tainted with the chemical.
10 Sept: 14 babies reported ill in Gansu province. Cases reported around China
13 Sept: Sanlu Group identified as source of contaminated powder milk. Production halted, 19 arrests
15 Sept: Beijing confirms two babies have died. Taiwan bans baby milk products
19 Sept: Death toll rises to four. Melamine found in ordinary milk from three well-known dairies.
22 Sept: Toll of ill babies rises to 53,000. Head of China's quality watchdog resigns. Twenty-two firms implicated in scandal.
23 Sept: Countries across Asia start to either test Chinese dairy products or pull them from shops
26 Sept: EU bans Chinese baby food with milk traces. Sales of popular sweet White Rabbit halted after tests detect melamine
29 Sept: Cadbury recalls products in Asia after tests find traces of melamine. Reports say 22 people arrested in Hebei province, suspected of introducing melamine into supply chain
China's dairy industry is on the brink of collapse, and importers of food products containing any Chinese milk products are being recalled from shops around the world.
More than a dozen Asian and African countries, plus the 27-member European Union, have taken steps to ban or otherwise limit consumption of Chinese milk-product imports. Laos, Mali and Niger on Monday became the latest to order such measures.
Besides the toll in mainland China, five children in Hong Kong, one in Macau, and four people in Taiwan have reportedly developed kidney stones after drinking tainted Chinese products.
The Chinese government says it is facing the spreading problem candidly, but rights organisations say that coverage of the scandal is controlled, and those trying to help victims are being harassed.
"China has tightened its grip on media freedom to contain rising nationwide outrage at tainted milk products," China Human Rights Defenders, a network of domestic and foreign human rights activists, said in an emailed report.
The rights group said the central government had ordered all Chinese media to toe the official line on the issue, thereby preventing exposure of "deep-seated problems in the system".
Melamine is used in making plastics and is high in nitrogen, which makes products appear to have a higher protein content.
Health experts say that ingesting small amounts does no harm but sustained use can cause kidney stones and renal failure, especially among the young.