China is vowing to stop tainted milk from reaching export markets, amid a growing scandal at home that has killed four children and sickened more than 50,000 others.  Stephanie Ho reports from Beijing.    

In this photo released by China's Xinhua News Agency, a nurse takes care of sickened child at Hebei People's Hospital in Shijiazhuang, 16 Sep 2008

Baby formula and other milk products have been pulled from stores across China because authorities have found milk contaminated with melamine.  The industrial chemical causes kidney stones and can lead to kidney failure.

Four Chinese infants have already died from drinking toxic milk products and more than 53,000 others have been made sick.

The widening scandal has already claimed resignations from several officials, including Li Changjiang, the head of China's top quality inspection agency.

His resignation comes one year after he and the Chinese government promised to overhaul the system in response to a series of product safety scares.

One Beijing resident, Zhao Yan, says the punishments so far are not enough.

Zhao says the Hebei provincial governor and the province's Communist Party secretary should also take responsibility and step down.

Hebei is home to the country's largest producer of powdered milk, Sanlu Group, which is at the center of the melamine contamination scandal.  The chemical was also found in products from 21 other Chinese dairy companies. 

Other countries also are threatened by contaminated Chinese milk products.  Markets that have banned or recalled Chinese milk products include Brunei, Hong Kong, Japan and Singapore.

Chinese foreign ministry spokeswoman Jiang Yu said China has, in her words, "comprehensively" informed the World Health Organization and "relevant" governments.

Jiang says the Chinese government is conducting a thorough investigation and will keep in contact with food safety authorities in other countries.

Chinese shopper looks at a notice posted on Mengniu milk products at a Wal-Mart store in Beijing, 21 Sep 2008

Meanwhile, Hong Kong share prices for one of China's biggest milk producers, Mengniu Dairy, plummeted almost 60 percent after its products were found tainted with melamine.

Francis Lun, of Hong Kong's Fulbright Securities, says the Chinese dairy industry has basically collapsed.

"Nobody is drinking milk anymore and nobody is buying any dairy products," Lun said.  "It will take a long time before the dairy industry is normal again and Mengniu Dairy can supply the milk products that it has been supplying."

The official Xinhua news agency says the Sanlu Group dairy company knew as early as June that there were problems with its milk powder.

But the company only reported the melamine poisonings to local authorities in August.  Local authorities, in turn, only reported the contamination problem to the central government in early September, after the Beijing Olympics.