Traces of melamine were found in nearly 12 percent of milk powder products that were tested in the country, Chinese authorities confirmed. This is one of the biggest food contamination scandals that has so far sickened 53,000 children and killed four.

As China marked its national day, President Hu Jintao said lessons must be learned from the scandal over tainted milk that has soiled China's reputation and led to a series of bans or curbs on its dairy exports worldwide.

The Anglo-Dutch company Unilever became the latest big-name brand to recall some Chinese products, taking Lipton milk tea powder off shelves in Hong Kong and Macau after tests showed them contaminated with melamine .

The chemical is normally used in making plastic, but can make watered-down milk look richer in protein than is actually the case.

"Food safety is directly linked to the well-being of the broad masses and the competence of a company," Hu said during a tour Tuesday of dairy companies in eastern China's Anhui province, the Xiinhua news agency reported.

"Chinese companies should learn from the lessons of the Sanlu tainted milk powder incident," he said, referring to Sanlu Group whose toxic baby formula was at the origin of the crisis.

Hu urged companies to strengthen management and food safety checks and make sure their products were safe for consumers, according to Xinhua.

Since the scandal first erupted last month, an increasing number of dairy products have been found to contain the industrial chemical melamine.