BEIJING  - Six children were killed Friday and about two dozen injured in an accident and stampede at a primary school in southwestern China, authorities and reports said.

Two of the injured were in serious condition, according to a statement on a news portal run directly by the municipal government in the city of Kunming.

The Yunnan Information Daily, a local newspaper, reported that the incident occurred when some children punched two three-metre (nearly 10 feet) tall mattresses causing them to fall on top of other children.

In the aftermath, children rushed down a stairway, causing some to fall and be trampled, it added.

The deaths of the six children, all in grades one and two - meaning they were likely six or seven-years-old - were announced in a press release from a district propaganda department in the city of Kunming.

The incident occurred Friday afternoon at Kunming’s Mingtong Primary School, authorities and reports said.

The injured were being treated at two local hospitals, the official Xinhua news agency said. Calls made to the Kunming municipal government went unanswered Friday afternoon.