US President Donald Trump has disputed Puerto Rico's official death toll of 3,000 from last year's Hurricane Maria and accused Democrats, without providing evidence, of inflating the figures to make him look bad.

"3000 people did not die in the two hurricanes that hit Puerto Rico … as time went by it did not go up by much," Mr Trump wrote on Twitter.

"Then, a long time later, they started to report really large numbers, like 3000."

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Puerto Rico's governor raised the US territory's official death toll from Hurricane Maria from 64 to 2,975 after an independent study found the number of people who succumbed in the aftermath had been severely undercounted.

The tweets drew the ire of Carmen Yulin Cruz, the mayor of San Juan, who has sharply criticised both the President and his administration's response to the storm last year that devastated the US territory.

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"This is what denial following neglect looks like: Mr Pres in the real world people died on your watch. YOUR LACK OF RESPECT IS APPALLING!" she wrote on Twitter.

She added another tweet saying the US leader was "delusional, paranoid and unhinged from any sense of reality."

It's not the first time Mr Trump and Ms Cruz have exchanged taunts on social media, with the San Juan leader having been critical of slow pace of the relief effort by federal authorities at the time of the category-four hurricane.

Mr Trump's tweets came as Hurricane Florence approaches the Carolina coast and is expected to cause dangerous flooding and tornadoes.

Forecasters at the National Hurricane Centre warned that Florence remains deadly because of its size and slow forward speed, even if its top sustained winds have dropped it to Category 2 status as a hurricane.

In all, an estimated 10 million people live in areas expected to be placed under a hurricane or storm advisory, according to the US Weather Prediction Centre.

Hurricane Maria caused widespread damage to Puerto Rico, leading to the death of nearly 3,000 people. ( Reuters: Carlos Garcia Rawlins )

Reuters/AP