Sanlu Group, a leading Chinese dairy producer, said it had found in its self-check that some of its baby milk powder products were contaminated by melamine, also known as tripolycyanamide.

The contaminated powder was all produced before Aug. 6 this year, and about 700 tonnes were in circulation, the corporation said. It has decided to recall all the baby milk powder it had produced before that date.

This year, at least 59 kidney stone cases in infants were reported in northwestern Gansu and other provinces. Investigations showed that most of the baby patients had drunk the Sanlu formula.

At least one baby in Gansu had died as a result of kidney stones.

Health experts said tripolycyanamide, a chemical raw material, could lead to stone formation in the urinary tract of the human body.

China's Health Ministry, after investigation, also said it strongly suspected that Sanlu, based in Shijiazhuang of Hebei Province, had produced milk powder contaminated by the chemical.

The ministry has launched a nationwide investigation into infant cases that might have resulted from drinking the formula, and is organizing experts to work out schemes for treatment.

China's quality watchdog, the Administration of Quality Supervision, Inspection and Quarantine, has also sent a team to Sanlu to investigate the cause of the accident, and meanwhile carried out nationwide check of other baby milk powders' quality.

The Health Ministry urged the public to immediately stop consuming the formula and go to doctors if they find babies who have drunk it have difficulty in urinating.

It also directed local medical facilities to immediately report similar cases and has notified the development to the World Health Organization and relevant countries.

Any new development of the case would be publicized immediately, the ministry promised.