BEIJING - Chinese police have arrested the head of a kindergarten and her associate over the deaths of two children from a rival school who drank yoghurt injected with rat poison, state media have said.

The five-year-old and six-year-old sisters from a village in northern Hebei province died after consuming the yoghurt drink which they had found in a bag on a roadside while walking to school with their grandmother.

Kindergarten director Shi Haixia and her associate Yang Wenming "admitted that they injected the poison into the yoghurt and left it on the street", the China Daily reported yesterday. "Their intention was to damage the reputation of the rival kindergarten."

The drink contained a strong rat poison called tetramine, the China Daily said, citing a local security official.

The pair were arrested on Wednesday, the day the younger girl died after a week in hospital. The six-year-old died within hours of consuming the yoghurt drink on April 24.

Recalling what happened that day, the girls' grandmother Ren Shuting told the Hebei Youth Daily newspaper that she had picked up the bag containing some books, pencils and a bottle of yoghurt drink and taken it home with her.

When the girls returned home from kindergarten in the afternoon, they drank the yoghurt.

"The girls asked me, 'Grandma, why is this yoghurt so bitter?'" the newspaper quoted Madam Ren as saying.

She tasted it but thought nothing of it.

Later that afternoon, while playing outside the house, the younger girl fell to the ground and started twitching and foaming at the mouth, followed by her elder sister who was in the house.

The girls were rushed to hospital, but the older girl died on the way there.

Madam Ren was hospitalised for several days.

The China Daily cited a professor of primary education as saying that the murders exposed the problem of unregulated private schools that had sprung up to compensate for the lack of adequate public kindergartens.

"This is an extreme case showing the lack of supervision in pre-school education," it quoted Professor Yuan Ailing of East China Normal University as saying.

Many children in the countryside are cared for by their grandparents because their parents have left for cities to find work.

AGENCE FRANCE-PRESSE