More than 100 children in the Indian state of Bihar - home to some of the country's worst health indicators - have now been killed by a brain virus potentially linked to lychees, officials said.

The northern state, one of India's poorest and home to almost 100 million people, is also in the throes of a major heat wave that is the country's second-longest on record and has so far claimed 78 lives.

Bihar has been struggling with an outbreak of Acute Encephalitis Syndrome (AES) since the start of June.

Eighty-five children have now died in the state's biggest government-run hospital - the Sri Krishna Medical College and Hospital - in the city of Muzaffarpur - and 18 others at a private facility, according to municipal officials cited by the Press Trust of India.

Most of the victims had suffered a sudden loss of glucose in their blood, health official Ashok Kumar Singh told AFP.

Such outbreaks have happened annually during summer months in the same districts since 1995, typically coinciding with the lychee season.

Several years ago, US researchers had said the brain disease could be linked to a toxic substance found in the fruit.

They also said more study was needed to uncover the cause of the illness, which leads to seizures, altered mental state and death in more than a third of cases.

TV channels showed distraught parents sitting next to their children, several of whom were cramped on one bed.

A doctor told a local TV channel that the SKMCH was ill-equipped to handle the rush of patients, most of whom were wheeled in semi-conscious.

Known locally as Chamki Bukhar, the disease claimed a record 150 lives in 2014.

Outbreaks of neurological illness have also been observed in lychee-growing regions of Bangladesh and Vietnam.

Large parts of India are reeling from drought

Meanwhile, the state has been experiencing temperatures around 45 degrees for several days, and authorities have imposed curfew-like restrictions.

Severe heat there has killed 78 people - most of them aged above 50 - across three districts since Saturday afternoon, local official Sandeep Kumar told AFP.

More than 130 others were undergoing emergency treatment for heatstroke in various hospitals.

Authorities in Gaya district - which has borne the brunt of the heat wave - invoked an Indian law to prohibit residents from going outdoors for non-essential work.

The district magistrate also banned construction work and any outdoor programme between 1am and 4pm.

Heatstroke is usually caused by prolonged exposure to sun or from physical exertion in high temperatures.

Daytime temperatures across large parts of India have hovered above 40 degrees for the past 32 days, just one short of a record 33-day period in 1988.

Temperatures touched 50.3 degrees in the town of Churu in the northern desert state of Rajasthan recently, just below India's record of 51 degrees.

Heatstroke has left more than 36 people dead in southern India in recent weeks. Large parts of India are also reeling from drought, with annual monsoon rains late in coming.