Thick smoke is seen rising from the campus of a school in Tianshui City, northwest China's Gansu Province, as the school is holding a fire drill on Friday, September 18, 2015. [Photo: people.com.cn]

More than 150 students are in hospital, 10 of them in a critical condition, after inhaling smoke from smoke guns used during a fire drill on Friday at their school in northwest China's Gansu Province.

The 165 students of the junior middle school in Tianshui City are at four hospitals, with 90 of them under less intense medical observation, according to a statement issued by the Tianshui government on Monday.

A Sunday statement said only nine students had been hospitalized after the incident.

The government said 412 students participated in the drill, which used smoke guns to simulate fire. Many of them began to cough and vomit after inhaling the smoke.