Problematic milk was suspected to have sickened 67 primary school students in northwest China's Shaanxi Province Friday, in the third dairy-related campus food poisoning case of the week.

The students suffered stomachache and vomiting after a breakfast consisting milk and eggs at Xushui Township Primary School in Hanzhong City Friday, said principal Ji Yanwen.

The school has more than 450 students. "Fortunately many of them did not choose milk for breakfast," Ji said.

The students were sent to a local clinic for treatment immediately. As of Saturday morning, nine serious cases had been transferred to the Yangxian county hospital. The other 58 students stayed at the clinic for treatment and observation, said Ji.

The county's quality inspection authorities had taken samples of the milk for lab work.

Meanwhile, local authorities in charge of health, food and drug and market regulation had launched a joint investigation.

Ji said the "Yili" brand milk, packed in 250-ml plastic bags, was manufactured on April 18 or 19. He was not certain whether it had outlived its shelf life by Friday.

The shelf life of milk on sale in Chinese market ranges between 24 hours to six months, depending on the different ways of sterilization and packing.

Meanwhile, authorities in Shaanxi are still investigating a similar incident in Zhouzhi County, where 18 students fell ill after drinking "Mengniu" brand milk on Thursday. By Saturday, all were discharged from hospital.

Mengniu and Yili are among China's leading dairy producers though both were involved in the melamine-tainted milk scandal of 2008.

Both brands are now sold to schools under a national nutrition project for minors to "drink more milk and become healthier."

The two companies have not commented on the cases.

Shaanxi's local brand, privately-owned Renrengao, was suspended operation after more than 200 students were sickened in Mianxian and Xunyang counties early this week.

The milk was found to contain excessive amounts of harmful E.coli and other bacteria, according to a local health authority report.

Investigators from the local quality inspection and food and drug authorities suspected the problems were caused during production and packing, but a company executive assumed the milk was contaminated during distribution.

"Milk packages may have been broken in transport so the milk went stale," said Liu Shuangbao, deputy manager of Renrengao Dairy Co. Ltd.