Lightning has killed at least 70 people, mostly farm labourers working in fields, in eastern India this week as a much anticipated summer monsoon advances across the country.

An official in the eastern state of Bihar said on Wednesday that at least 56 people had been killed by lightining since Monday. Another 24 were injured by severe thunderstorms and monsoon rains in at least 14 districts of the state.

Authorities in the neighbouring state of Uttar Pradesh said at least 20 people had been killed over the past two days.

The Bihar disaster management official Vyasji, who uses only one name, said that scores of cattle also died after being struck by lightning.

READ MORE: Monsoon advances across India

The dead included at least eight shepherds out grazing their sheep, said Vyasji.

Lightning strikes are common during India's monsoon season, which runs from June to September. However, the past day's toll in Bihar was unusually high.

Lightning kills thousands of Indians each year, most of them farmers working the fields.

More than 2,500 people were killed by lightning in India in 2014, according to the National Crime Records Bureau, the most recent figures available.