India's coronavirus death toll is up to 10 times higher than the nearly 415,000 fatalities reported by authorities, likely making it the country's worst humanitarian disaster since independence, a United States research group has said.

The Centre for Global Development study's estimate is the highest yet for the carnage in the South Asian nation of 1.3 billion people, which is emerging from a devastating surge partly fuelled by the Delta variant in April and May.

The study — which analysed data from the start of the pandemic to June this year — suggested that between 3.4 million and 4.7m people had died from the virus, according to an AFP report.

“True deaths are likely to be in the several millions, not hundreds of thousands, making this arguably India's worst human tragedy since partition and independence,” the researchers said.