NEW DELHI (DPA) - At least 100 children have died of encephalitis in India's eastern state of Bihar, where health authorities are grappling with a surge of cases in the disease that causes the brain to swell.

The deaths, reported since early June, occurred mostly in the Muzaffarpur district, considered the epicentre of the outbreak, principal health secretary Sanjay Kumar said.

Eighty-three children had died in the state-run Sri Krishna Medical College and Hospital in Muzaffapur, hospital superintendent Sunil Kumar Shahi said, adding 17 more had died at a private hospital in the city.

The victims were all children under the age of 7, he said.

Some 291 more children from Muzaffarpur and nearby districts have been admitted to hospital, he added. The number of deaths spiked over the past few days as the toll had been put at 67 on Friday.

Encephalitis is a brain inflammation, mostly caused by viral infection. The risk of disease increases during summer and rainy seasons between June to October.

Kumar said there could be multiple reasons behind the outbreak of encephalitis.

Some health officials cited hypoglycaemia, a condition caused by very low level of blood sugar and electrolyte imbalance, as the main reason for deaths.

Health professionals suspect the deaths to be linked to toxins present in lychee fruits, which had been detected in urine samples of patients, local media reported.

Children with low blood sugar levels caused by malnutrition could develop encephalitis after eating a large amount of the fruit which is grown in the region.

The Japanese encephalitis virus is the most common cause of encephalitis syndrome in the country, causing up to 35 per cent of the cases, the reports said.