China has launched a 10-day emergency crackdown on tainted milk products after several were found creeping back onto the market despite a massive scandal that sickened hundreds of thousands of children in 2008.

No one knows how many tainted milk products are still on the market, a member of the country's food safety committee, Chen Junshi, was quoted as saying by the China Daily newspaper Tuesday.

The sweep that started Monday comes after milk products tainted with the industrial chemical melamine were pulled from shelves in Shanghai and the provinces of Shaanxi, Shandong, Liaoning and Hebei, the state-run Xinhua News Agency said. Some had been recalled in the previous scandal and repackaged.

At least six children died and more than 300,000 fell sick in the 2008 scandal, where melamine, normally used in making plastics and fertilizer, was added to watered-down milk to fool inspectors testing for protein and increase profits.

At the time, China promised sweeping changes for the country's food safety.

But health concerns peaked again early this year after authorities in Shanghai said they secretly investigated a dairy for nearly a year before announcing it had been producing tainted milk products.

The case was especially troubling because Shanghai Panda Dairy Co. was one of the 22 dairies named by China's product safety authority in the 2008 scandal, with its products having among the highest levels of melamine.

This time, China is again promising a thorough crackdown. "All melamine-tainted milk products will be found and destroyed," Xinhua quoted Health Minister Chen Zhu as saying over the weekend.

Chinese authorities won't be able to get every tainted packet, but it's still a good move, said Victoria Sekitoleko, representative for the U.N.'s Food and Agriculture Organization in Beijing.

"For now, it's best to go in right away and get it off the shelves," she said. "These traders are not about to give up, so officials will have to be vigilant all the time."

In other recent cases, officials in late January said tainted dairy products from three companies were pulled from more than a dozen convenience stores around the southern province of Guizhou. Officials said products recalled during the previous scandal somehow made it back to the market.

Again, one of the companies, Laoting Kaida Refrigeration, was among companies named in the original 2008 scandal.

In December, the general manager of a dairy in northern Shaanxi province and two employees were accused of producing and selling more than 5 tons of tainted milk powder. The deputy head of the regional public security department told Xinhua that none of the powder reached the market.