GANDAMAN, India (AFP) - The mother of three children who died after eating a poisoned lunch in an impoverished part of India told on Thursday how she had sent them to the school only so they could get a free meal.

As the death toll from the tragedy at a primary school in Bihar state rose to 23, the police stepped up their investigation, including the possibility the free lunches given to the children were deliberately poisoned.

The state government meanwhile revealed plans to pay compensation of nearly US$4,000 (S$5,030) to families who lost their loved ones, but the announcement did little to assuage the sense of grief or anger among parents.

"My children always liked eating at the school and I was happy that at least they were getting one square meal every day, but I never dreamt that it would end up killing them," said Ms Sanjudevi Mahato as she wept for the loss of three of her four children.

"My husband is bedridden. We have no food at home and it was only to ensure that my children got at least some food that I sent them to the school," she told AFP at a hospital in the state capital Patna.

Ms Mahato was speaking as her fourth child, an eight-year-old girl called Kajal, received treatment in hospital from the effects of the poison.

"She only survived because she could detect the pungent smell in the food and refused to eat."

Ms Jankidevi Kumar, whose five-year-old son Ashok also died from food poisoning, said it was not the first time that there had been complaints about the food at the primary school in the village of Gandaman.

"My elder son kept complaining that the food stank... it tasted bitter but the headmistress insisted that all the children should eat it," she told AFP outside the school grounds in Gandaman.

The 23 children, aged four to 12, died after eating lentils, vegetables and rice cooked at the school on Tuesday. Initial tests have shown the food may have been contaminated with insecticide.

Some 30 children are still being treated for food poisoning, although doctors say their condition is not life-threatening.

"The death toll has risen to 23," Bihar state education secretary Amarjeet Sinha told reporters.

"It seems like a deliberate case of poisoning and we are expecting a forensic report to confirm the cause behind the incident."

No one has yet been arrested over the deaths, although the police conducted raids on Wednesday night across the local district of Saran.

They raided the home of the headmistress Meena Kumari, who fled after the children started dying on Tuesday, a senior officer said on condition of anonymity.

State Education Minister P. K. Shahi said on Wednesday the police were probing whether the food was accidentally or possibly deliberately poisoned.

The minister said the cook complained to the headmistress about the smell of the oil before the meals were served on Tuesday but the headmistress dismissed her concerns.

The tragedy has sparked panic elsewhere in Bihar, with reports from dozens of schools of children dumping their meals in bins or refusing even to touch them.

"Parents have warned their children to not even touch the meal served in the school," Mr Lakshmanan, a senior state government official who uses only one name, told AFP.

"Some of the students dumped the lunch in school dustbins and we are trying to convince everyone that the tragedy will not be repeated," said Mr Lakshmanan, director of the midday meal scheme in Bihar.

India's state governments run the world's largest school feeding programme involving 120 million children. Bihar is one of India's most populated and poorest states.

Educators see the scheme as a way to increase school attendance, in a country where almost half of all young children are undernourished. But children often suffer from food poisoning due to poor hygiene in kitchens and occasionally substandard food.

The authorities have instructed all teachers and cooks in the state to first taste free lunches before serving them to children.

"We will have to make parents believe that midday meals provide nutrition and are not meant to kill students," said Mr Lakshmanan.