Bad milk or eggs are suspected to have sickened 42 primary school students Friday in Xi'an, capital of northwest China's Shaanxi Province, the fifth campus food poisoning case in the province since late April.

The students suffered stomachaches and nausea, symptoms of mild food poisoning, after eating a breakfast of milk and eggs at a Ziwu Street Central Primary School, in Chang'an District, said Wang Runnian, a government spokesman.

The children recovered and returned to class Friday afternoon after being treated at a district clinic.

None of the children displayed serious symptoms like vomiting or diarrhea.

The school sealed the milk and eggs to assist health and disease control authorities' investigations.

It was the fifth food poisoning case since April 19. The first two occurred in Mianxian and Xunyang counties in Shaanxi Province.

About 250 students were sickened in all five cases.

Investigators said the first two cases were caused by lax packaging of the milk, a local brand named "Renrengao," which has suspended operations.

Investigation results of other cases are yet to be released.

Since last fall, many areas of Shannxi have carried out an "egg and milk project." Through it, boarding students eat one egg and one bag of milk for a breakfast partially funded by the government.

Provincial health and education departments jointly released a regulation earlier this month to ensure food safety for primary and middle school students. The regulation will take effect on June 1.