NEW DELHI, India - 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 US research group said Tuesday.

The Center 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 analyzed data from the start of the pandemic to June this year -- suggested that between 3.4 million and 4.7 million people had died from the virus.

"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.

After the sub-continent's partition in 1947 into mainly Hindu India and Muslim-majority Pakistan, sectarian bloodshed killed hundreds of thousands of people. Some estimates say up to two million died.

India's official death toll of just over 414,000 is the world's third-highest after the United States' 609,000 fatalities and Brazil's 542,000. Experts have been casting doubt on India's toll for months, blaming the already overstretched health service.

Several Indian states have revised their virus tolls in recent weeks, adding thousands of "backlog" deaths.

The Center for Global Development report was based on estimating "excess mortality", the number of extra people who died compared with pre-crisis figures.

The authors -- who included Arvind Subramanian, a former chief government economic adviser -- did this partly by analyzing death registrations in some states as well as a recurring national economic study.

They also compared surveys of the spread of COVID-19 in India with international death rates.

The researchers, which also included a Harvard University expert, acknowledged that estimating mortality with statistical confidence was difficult.

"(But) all estimates suggest that the death toll from the pandemic is likely to be an order of magnitude greater than the official count," they said.

- 'Speculative' -

Christophe Guilmoto, a specialist in Indian demography at France's Research Institute for Development, this month estimated that the death toll was nearer 2.2 million by late May.

India's death rate per million was nearly half the world average and Guilmoto said "such a low figure contradicts the apparent severity of a crisis that has struck most Indian families across the country".

Guilmoto's team concluded that only one coronavirus death in seven was recorded.

A model by the US-based Institute for Health Metrics and Evaluation estimated that the COVID-19 toll could be more than 1.25 million.

India's health ministry last month slammed The Economist magazine for publishing a story that said excess deaths were between five and seven times higher than the official toll, calling it "speculative" and "misinformed".

A World Health Organization report in May said up to three times more people had died around the globe during the pandemic -- from coronavirus or other causes -- than indicated by official statistics.