A forensic investigation into the food served to children at a school in north-eastern state of India has confirmed the presence of insecticide.

The food was served to children as part of the Indian government's Mid-Day Meal Scheme and resulted in the death of more than 20 students in the village of Gandamal in Bihar, sparking national outrage.

Media reports add the poison was detected in both the cooking oil used and in the mixture of rice and vegetables.

"It was observed that the peak area of the poisonous substance in the oil was more than five times in comparison to the commercial preparation used as a control. It is a matter of investigation that how it came into the food," Ravinder Kumar, Additional Director General (ADG) of Police in Bihar's capital Patna, told NDTV.

The main compound found was Monocrotophos, a pesticide used for agricultural purposes and known to be extremely toxic to human beings and animals. Earlier reports suggested the tragic incident was the result of mismanagement by school authorities, including the principal and teachers in charge of preparing food.

Meanwhile, only days after the Bihar school disaster, reports of similar cases have come flooding in from other parts of the country.

Last midweek, 170 children reportedly fell ill after lunch at a school in Neyveli, in the southern Indian state of Tamil Nadu. And 30 more students have fallen ill in Bemetara district in Chhattisgarh state, after lunch on Saturday.

"Thirty students of a Primary school at Maugaon village complained of stomach pain and vomiting after eating the mid-day meal, following which 25 of them were rushed to Bemetara district hospital," a government official told the Press Trust of India (PTI).

"The condition of these 25 children is said to be out of danger and other kids were sent back home after a preliminary treatment at the village," the official continued, adding, "... cause can be ascertained only after the probe ends and stern action will be taken against those found guilty."

Local officials have ordered an investigation into the incident and the government has sent authorities to help with the matter.