BEIJING (AFP) - China has detained six people after a stampede at a primary school saw four children die in an incident in which they were crushed against a locked gate, a local official said on Thursday.

The six - including the school principal - were among 12 people being investigated for the accident in Laohekou, in the central province of Hubei, a city official said.

The stampede happened early morning Wednesday when large numbers of pupils left their school dormitory, on the fourth floor, and attempted to exit the block.

The ground floor gate was closed, resulting in the crush.

Another seven pupils were injured in the stampede, although the ages of those involved was unclear.

"Six people have been sent to judicial organs from a total of 12 people who are under investigation," a local government official surnamed Zhong said.

The accident has provoked anger on China's Internet message boards.

"The children are the hope of the motherland," said one poster on Sina Weibo, China's version of Twitter. "All I have seen is education officials being interviewed, but I want to see sentencing."

Safety at Chinese schools has become a major issue in recent years.

A stampede in a primary school in the small city of Aksu in China's far western Xinjiang region left 41 children hospitalised in 2010.

A year earlier, eight students were killed in a stampede at a school in the central province of Hunan.