XINING - A total of 548 students fell ill after consuming milk and biscuits in schools in Northwest China's Qinghai province on Wednesday, local authorities said.

The food poisoning incident involved primary and middle school pupils in seven townships in Hui-Tu autonomous county of Datong, and about a dozen of them were in serious condition, said a statement from the county's publicity bureau.

Students showed symptoms of vomiting, dizziness and headache after having the extra meal provided by schools as a nutritious supplement between breakfast and lunch, the statement said.

A middle school student surnamed Ma said that the milk provided this morning had "a strange smell," and that she suffered nausea and headache minutes after drinking it and then fainted in the classroom.

Fifty-six of the county's 72 primary and middle schools have been affected by the food poisoning, a local official said.

Initial probes have confirmed the milk or biscuits as the cause of the food poisoning, said Wu Hai, secretary of the county's committee of the Communist Party of China.

The local food and drug quality watchdog has sealed off all the food in the same batch of the "nutritious meals," and samples have been sent to provincial authorities for inspection.

The Datong government started providing free "nutritious meals" to 450,000 rural students in primary and middle schools last spring in a campaign to improve nutritional conditions.

Some of the affected students resumed classes in the afternoon.