US President Donald Trump has disputed Puerto Rico's official death toll of 3,000 from hurricanes last year and accused Democrats of inflating the figure, which actually came from an independent academic study.

Mr Trump bristled at criticism of his administration's handling of the Puerto Rico disaster as another hurricane, Florence, moves toward the southeastern United States.

Mr Trump said Democrats inflated the number of dead in Puerto Rico "in order to make me look as bad as possible" but he did not provide evidence.

In a tweet, Mr Trump said: "3000 people did not die in the two hurricanes that hit Puerto Rico. When I left the Island, AFTER the storm had hit, they had anywhere from 6 to 18 deaths. As time went by it did not go up by much. Then, a long time later, they started to report really large numbers, like 3000."

Puerto Rico was recovering from Hurricane Irma before Hurricane Maria hit in September 2017, destroying roads and bridges and leaving much of the Caribbean island without electricity for months.

The death toll from Maria, the most powerful storm to hit there in almost a century, was raised last month from 64, a number widely discounted as far too low, to 2,975.

That number was produced by public health experts at George Washington University in Washington in a report commissioned by the US territory's governor, Ricardo Rossello.

The study found that those deaths could be attributed directly or indirectly to Maria from the time it struck in September 2017 to mid-February of this year.

The report compared predicted mortality under normal circumstances and deaths documented after Maria.

Researchers said they adjusted for factors that could account for fluctuations in mortality, most notably the displacement of some 241,000 people who fled the island in the immediate aftermath of the storm.

The top Republican in Congress, House of Representatives Speaker Paul Ryan, said he had no reason to doubt the official death toll.

"Casualties don't make a person look bad, so I have no reason to dispute these numbers," he said.

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Mr Rossello said Puerto Ricans "do not deserve to have their pain questioned" and backed the study.

"We left this analysis to the scientists and experts, recognising that there would be many challenges, because we wanted to have a powerful and independent voice to minimize the uncertainty," he said in a video on Facebook.

The emergency response to Maria became highly politicised as the Trump administration was criticised as being slow to recognise the extent of the devastation and too sluggish in providing disaster relief to Puerto Rico, an island of more than three million residents.

Mr Trump touted the federal government's response as "an incredible, unsung success" this week.

His tweets drew the ire of Carmen Yulin Cruz, the mayor of Puerto Rican capital San Juan who has sharply criticised both the president and his administration's response to Maria.

"This is what denial following neglect looks like: Mr Presin the real world people died on your watch. YOUR LACK OF RESPECT IS APPALLING!" she wrote on Twitter before calling Mr Trump delusional and unhinged from reality.

On Wednesday, Puerto Rican officials said about 20,000 pallets of water bottles shipped to Puerto Rico after Maria went unused and became too contaminated to drink.