Hurricane Maria, which pummeled Puerto Rico in September 2017, is likely responsible for the deaths of more than 4,600 people, some 70 times higher than official estimates, US researchers said on Tuesday.

The government-provided death toll stands at just 64, but experts say an accurate count was complicated by the power outages and widespread devastation wreaked by the storm, which caused $90 billion in damages and is ranked as the third costliest cyclone in the United States since 1900.

Earlier independent investigations have put the true toll at closer to 1,000.

But the latest estimates, compiled by researchers at Harvard University, came back far higher - at 4,645 deaths from the day of the storm, September 20, until December 31, 2017.

For comparison, the death toll from 2005's Hurricane Katrina - the costliest hurricane in US history - was far lower, and estimated at 1,833.

Most deaths after Hurricane Maria hit Puerto Rico are blamed on interruptions in medical care due to power outages and blocked or washed out roads, said the report published in the New England Journal of Medicine.

"Approximately one-third of post-hurricane deaths were reported by household members as being caused by delayed or prevented access to medical care," said the report.