Hurricane Dorian is bearing down on the US territory of Puerto Rico, where residents prepared for a direct hit, the first since the island was ravaged two years ago by Hurricane Maria.

Even before the storm struck, an 80-year-old man died in a fall from a ladder while fixing a roof in a San Juan suburb, police said.

US forecasters said Dorian was upgraded to a hurricane from a tropical storm near St Thomas in the US Virgin Islands - just east of Puerto Rico - and is expected to make landfall in populous eastern Puerto Rico in the next 24 hours.

The latest path also puts Dorian on a trajectory to strike the Atlantic coast of Florida - which declared a state of emergency - or Georgia by the weekend.

"Heavy rainfall over portions of Puerto Rico and the Virgin Islands could produce flash flooding through Thursday morning," the US National Hurricane Center said, adding that Dorian is gradually moving away from the northeastern Caribbean Sea.

"Dorian is forecast to strengthen and become a powerful hurricane during the next few days over the Atlantic waters."

Residents of the seaside town of Fajardo, hard hit by Maria in 2017 and now directly in Dorian's path, filled up their vehicles with fuel and stocked up on basic necessities.

Roads were nearly empty, businesses were closed, and people sheltered in their homes.

A man prepares a store for the arrival of Dorian in Humacao, Puerto Rico 

Dorian also developed into a political storm.

President Donald Trump declared a state of emergency in Puerto Rico authorising federal assistance even while alleging the island is "one of the most corrupt places on earth.

"Their political system is broken and their politicians are either Incompetent or Corrupt," he said on Twitter.

Former governor Ricardo Rossello was forced to resign last month in part because of criticism over his handling of the emergency created by Maria.

San Juan mayor tells Trump to 'keep his mouth shut'

Puerto Rico's new governor, Wanda Vazquez, said the island was better prepared this time to respond to any contingency.

San Juan Mayor Carmen Yulin attacked President Trump, telling CNN that the president "continues to try to belittle the people of Puerto Rico."

She told him "to keep his mouth shut" and let people do the work needed to save lives.

Evacuations were getting under way, starting with people living in some of the 30,000 Maria-damaged homes that still have blue tarps instead of roofs, Carlos Acevedo Caballero, head of the local emergency management agency, told reporters.

Maria was a major category four hurricane on a scale where five is the maximum. It shattered the island's already shaky power grid, overwhelmed public services and left many residents homeless.

A study accepted as valid by the government, which initially put the death toll at 64, estimated that nearly 3,000 people died as a result of the hurricane and the months of disruption that followed.

Dorian, though far less powerful, looms as the first major test of the island's halting recovery.

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As of 9pm tonight, the storm was about 70kms west of St Thomas, with maximum sustained winds that had increased to 129 kms per hour, the National Hurricane Center said.

Moving northwest, it is forecast to dump 10-15 centimetres of rain on Puerto Rico and the Virgin Islands, and up to 20cm on the coastal southeastern United States.

Dorian is expected to move away from the US and British Virgin Islands "during the next several hours" and then over the Atlantic well east of the southeastern Bahamas tomorrow and on Friday.

Florida Governor Ron DeSantis urged all residents on the east coast of his state to prepare for impact.

"Today, I am declaring a state of emergency to ensure Florida is fully prepared for Hurricane Dorian," he said in a statement.