The announcement came as quality inspection officials revealed that about 20 per cent of dairy companies so far tested nationwide had been found to have sold products tainted with melamine.

The General Administration of Quality Supervision, Inspection and Quarantine added that of 22 companies found to have produced contaminated baby formula, two had exported the product to other countries.

Melamine

Chinese authorities say melamine was put into the milk powder to make it appear the product had more protein.

The chemical, normally used to make plastics and glues, is rich in nitrogen, an element often used to measure protein levels, and so can be used to disguise diluted milk.

It is being blamed for causing kidney stones in the affected babies, a condition that is rare in infants but which causes a range of health risks.

Li Changjiang, the head of the watchdog body, said the two companies, Yashili and Suncare, exported to Myanmar, Yemen, Bangladesh, Burundi and Gabon.

He said that the two companies' products had been recalled but did not say whether the exported products had been contaminated with melamine.

The scandal, which has seen thousands of parents in southern China flood into Hong Kong to buy foreign-made milk powder, is the latest to rock China's food industry, which has been tarnished by a series of health scares in recent years.

Wednesday's developments also came as China brought down the curtain on the Paralympics - after also hosting the Olympics last month – with the scandal reviving an unflattering image of the country.

But Al Jazeera's Melissa Chan, reporting from Beijing, said the announcement and inspections by the Chinese government were encouraging signs that it was providing some transparency and getting serious about food safety. 

So far four officials have been sacked over the scandal in the city of Shijiazhuang, the base of the Sanlu Group, the top-selling dairy company first linked to the toxic milk.

Among those dismissed is Shijiazhuang's vice-mayor in charge of agriculture and the director of the city's food and drug watchdog.

Sanlu's chairwoman and general manager, Tian Wenhua, was also sacked.

Chinese officials last week ordered a nationwide check of all baby milk powder makers after Sanlu's problems began to emerge.

Sanlu, which is 43 per cent owned by New Zealand dairy giant Fonterra, last week halted production and announced a big product recall.

But New Zealand's prime minister said this week that local Chinese officials acted only after Wellington contacted the central government.