ST JOHN ISLAND, United States - Hurricane Maria closed in on the Virgin Islands and Puerto Rico on Wednesday as forecasters warned of a "potentially catastrophic" storm that has already killed at least eight people in the Caribbean.

The US National Hurricane Center said Maria was packing winds of up to 265 km/h as it approached the US and British Virgin Islands, which are still struggling to recover from devastating storm Irma.

Maria was set to remain "an extremely dangerous category 4 or 5 hurricane" as it pushed on to the US territory of Puerto Rico, the NHC said, warning of "large and destructive waves".

"Puerto Rico being hit hard by new monster Hurricane," tweeted US President Donald Trump.

"Be careful, our hearts are with you - will be there to help!"

Puerto Ricans were scrambling to board up windows and buy last-minute supplies as the storm loomed over the densely populated island of 3.5 million.

Locals in the US Virgin Islands reported horizontal rain and trees swirling in the wind.

"Very violent and intense right now as we have just begun to experience hurricane-force winds," said 31-year-old Coral Megahy, hunkered down on the St Croix island.

"We can hear debris banging on the aluminum windows as well."

Maria has already ripped through several Caribbean islands, leaving at least two people dead in the French territory of Guadeloupe and causing major damage in Dominica.

Culebra, Vieques and parts of the Dominican Republic are also on alert.

Puerto Rico has opened about 500 shelters with the capacity to take in 67,000 people, as Governor Ricardo Rossello warned residents to brace for "the worst storm of the last century".

"This is going to be an extremely violent phenomenon," Rossello said. "We have not experienced an event of this magnitude in our modern history."

"I'm not denying I'm scared," said Noemi Aviles Rivera, a 47-year-old schoolteacher who experienced Hurricane Hugo in 1989 and Georges in 1998.

"I feel worried because it's the first time I'll see a hurricane of this magnitude."

Rossello tweeted that more than 11,000 people had sought shelter already, with nearly 200 pets in tow.

On neighboring Dominica, Prime Minister Roosevelt Skerrit posted on Facebook that there were initial reports of "widespread devastation".

Communications to the tropical island of 73,000 people have been cut, and its airports and ports have been closed.

There had been fears that Maria could wreak fresh havoc on islands that were already flattened by maximum Category 5 hurricane Irma earlier in the month.

Early reports suggested St Martin, a French-Dutch island that was one of the worst hit by Irma with 14 dead, had escaped the worst this time around.

"Compared to Irma this was a breeze," Gordon Snow, editor of the Daily Herald newspaper, told the radio station Paradise FM.

Afp - Ap