China on Thursday fired the mayor of the northern city of Shijiazhuang, the base of the company involved in a scandal over contaminated milk powder that killed at least three infants and sickened more than 6,000.

Ji Chuntang was also sacked from his post as deputy head of the Shijiazhuang branch of China's ruling Communist Party, following the dismissal earlier this week of at least four other senior officials in the city, the government's Xinhua news agency reported.

Government officials said the Shijiazhuang-based Sanlu company, which supplies nearly a fifth of China's market with an inexpensive baby formula, knew about the contamination of milk powder with melamine since March but didn't order a national recall of the powder until this week.

Sanlu has made a public apology, and the Hebei provincial government, which administers Shijaizhuang, announced that more than 10,000 tons of baby formula recalled in the contamination scandal would be destroyed.

Three babies have died, 158 children have developed "serious kidney problems" and 6,244 infants were made ill by contaminated baby formula from Sanlu and other firms, Chinese officials said Wednesday.

Inspectors found melamine in 69 batches of milk powder made by 22 companies nationwide, said Li Changjiang, head of the State Administration of Quality Supervision, Inspection and Quarantine (AQSIQ).

The inspectors tested a total of 491 batches of milk products sold by all 109 Chinese companies licensed to produce baby formula milk powder, Li said.

Officials ordered a halt to the sale of the tainted products which included such well-known brands as Sanlu, Mengniu, Yili and Yashili.

Most of the babies treated in hospital had developed kidney stones and other urinary problems after consuming Sanlu milk powder for three to six months.

The AQSIQ found the melamine content in Sanlu brand reached 2,563 milligrams per kilogram, the highest among all the samples. In other samples, the range was from 0.09 to 619 milligrams per kilogram.

Police in the northern province of Hebei charged four people in connection with the Sanlu melamine contamination and detained 22 others, state media said.

The four arrested men are accused of adding melamine to their raw milk to boost its protein content before selling it to Sanlu, Xinhua said.

Melamine is used as a binding agent and coating for particle, fibre and laminated board. It is also used to make fertilizer.