Hurricane Maria slammed into Puerto Rico as a "potentially catastrophic" Category 4 storm that was set to become the most powerful to strike the US territory in almost 90 years.

An adviser to Dominican Prime Minister Roosevelt Skerrit says there have been seven confirmed deaths in the Caribbean country from Hurricane Maria.

Hartley Henry didn't give details about how the deaths occurred.

They raise the overall death toll to nine from the storm including two on the French island of Guadeloupe.

The New York Post reports thousands of Puerto Ricans hunkered down in shelters as Maria - packing sustained winds of 249kmh - made landfall as a Category 4 near Yabucoa, according to the US National Hurricane Centre.

The entire island, home to about 3.5 million people, is now without power

Governor Ricardo Rossello - who warned residents to brace for the "the worst storm of the last century" - tweeted that more than 11,000 people had sought refuge in about 500 shelters with 200 pets in tow.

"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."

Zinc roofs flew and windows broke as the storm bore down.

Those who sought shelter at a coliseum in San Juan were moved to the building's second and third floors, reported radio station WKAQ 580 AM.

Maria is expected to dump as much as 25 inches (63cms) of rain on parts of the island and bring storm surges of up to 3 metres, the NHC said. Officials feared that the heavy rain could unleash life-threatening mudslides.

The island of about 3.4 million people avoided a direct hit from Hurricane Irma, but the storm knocked out power for 70% of the territory and killed at least three people.

Maria was set to be the strongest hurricane to hit Puerto Rico since 1928, when the San Felipe Segundo hurricane killed about 300 people, according to the National Weather Service.

The last Category 4 hurricane landfall in Puerto Rico occurred in 1932.

"This is going to be catastrophic for our island," Grisele Cruz, who was staying at a shelter in the southeastern city of Guayama, told Reuters. "We're going to be without services for a long time."

More than 150 flights were cancelled at the Luis Munoz Marin International Airport, the main international airport in Puerto Rico, according to tracking service

As Maria approached, US President Donald Trump offered his support via Twitter.

"Puerto Rico being hit hard by new monster Hurricane. Be careful, our hearts are with you- will be there to help!" he said.

Later Wednesday and into Thursday, Maria will pass just north of the northeast coast of the Dominican Republic, the NHC said. The Turks & Caicos Islands and the southeastern Bahamas will also experience hurricane conditions.

It was too early to know if the storm will threaten the continental US as it churns northward in the Atlantic.

A few hours earlier, Maria passed west of St. Croix, home to about half of the US Virgin Islands' 103,000 residents, as a Category 5 storm.

Authorities expected to start assessing storm damage on St Croix at daybreak.

On Monday, the storm ploughed into Dominica, a mountainous country of 72,000 people, causing what Prime Minister Roosevelt Skerrit called "mind-boggling" destruction.

North of Dominica, the French island territory of Guadeloupe also was hit hard. Authorities told Reuters that one person was killed by a falling tree and at least two people were missing in a shipwreck.

Maria ties for the eighth-strongest storm in Atlantic history, when measured by wind speed.

Coming in second is Irma, which had 300-km/h winds and killed 38 people in the Caribbean and another 36 in the US earlier this month.