The growing number of cases in recent weeks challenges the government's earlier promise to overhaul its approach to food safety after hundreds of thousands of children in that scandal were sickened by milk products tainted with an industrial chemical. At least six children died.

Already, tainted milk products have recently emerged in China's largest city, Shanghai, and in the provinces of Shaanxi, Shandong, Liaoning, Guizhou, Jilin and Hebei.

China's 10-day emergency crackdown on the products is set to end Wednesday, and it was not clear whether it would be extended. The country's biggest holiday, the Lunar New Year, starts this weekend, and already some offices are closing and millions of people are going on vacation.

In the latest discovery, officials recalled more than 170 tons of milk powder tainted by the industrial chemical melamine and closed two dairy companies in the northern region of Ningxia, the China Daily newspaper reported Monday.

The report said officials seized 72 tons of the powder but were still looking for the rest, which had been repackaged by the Ningxia Tiantian Dairy Co. Ltd. and sold to factories in the neighboring region of Inner Mongolia and the bustling southern provinces of Guangdong and Fujian.

Dairy suppliers in the past have been accused of adding melamine, which is high in nitrogen, to watered-down milk to make it appear protein-rich in quality tests that measure nitrogen.

The report said the tainted powder should have been destroyed in the 2008 scandal, and that Ningxia Tiantian Dairy got it from an unnamed company as a debt payment.

"Our small companies were in total trust of their partners because they've been doing business and having good relations with them for a long time," Zhao Shuming, secretary-general of the Ningxia Dairy Industry Association, told The Associated Press. "They didn't expect those companies would hurt them."

China Daily quoted Zhao as saying many small dairies, including Ningxia Tiantian, don't have the technology to even test for melamine.

"Flaws in the previous system led to the current chaos. What if companies with tainted milk also hold back their stocks for this round of checkups and reuse them later, just like what's happening now?" the newspaper quoted him as saying.

Zhao spoke more carefully Monday, telling the AP, "We have strict checks and our client companies have strict checks too."

The 2008 milk scandal was China's worst food safety crisis in years. Chinese officials knew tracking and getting rid of the tainted milk products from the scandal would be difficult, and it didn't promise to destroy seized products itself.

Instead, it issued guidelines on how to destroy the tainted products, suggesting they be burned in large-capacity incinerators or that small amounts be buried in landfills. In the southern city of Guangzhou, however, the local government took over the disposal after one garbage company poured tainted milk into a city river.

China also issued a new food safety law last summer that places more responsibility on food producers to ensure their products are safe.