The strongest hurricane to hit Puerto Rico in over 80 years tore off roofs and doors, knocked out power across the entire island and unleashed heavy flooding Wednesday in an onslaught that could plunge the U.S. territory deeper into financial crisis.

Leaving at least nine people dead in its wake across the Caribbean, Hurricane Maria blew ashore in the morning in the southeast coastal town of Yabucoa as a Category 4 storm with winds of 250 km/h.

Forecasters said Wednesday evening that the storm is moving away from Puerto Rico's northwest coast. With that will come some relief from high winds, flooding and storm surge, the U.S. Hurricane Center said. But heavy rain will likely continue, and forecasters said there remains a risk of "catastrophic flash flooding" in some parts of the island.

The still-powerful hurricane is now moving toward the eastern Dominican Republic.

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As the Dominican Republic prepared for the storm, President Danilo Medina cancelled his speech before the UN General Assembly so that he could return home and co-ordinate preparations for Maria.

The hurricane centre said the maximum sustained winds were near 175 km/h on Wednesday evening, with stronger gusts.

"Some strengthening is forecast during the next day or two, and Maria could regain major hurricane status by Thursday."

'Together, we will rebuild'

In Puerto Rico, emergency officials expressed deep concern. 

"Once we're able to go outside, we're going to find our island destroyed," said Abner Gomez, Puerto Rico's emergency management director. "The information we have received is not encouraging. It's a system that has destroyed everything in its path."

Gov. Ricardo Rossello warned of heavy rains and flooding but urged people to have faith: "We are stronger than any hurricane. Together, we will rebuild." 

He later asked U.S. President Donald Trump to declare the island a disaster zone, a step that would open the way to federal aid.

There was no immediate word of any deaths or serious injuries, but initial reports suggest that the entire island is without power. 

As people waited it out in shelters or took cover inside stairwells, bathrooms and closets, Maria brought down cell towers and power lines, snapped trees and unloaded at least 50 centimetres of rain.

Widespread flooding was reported, with dozens of cars half-submerged in some neighbourhoods and many streets turned into rivers. People calling local radio stations reported that doors were being torn off their hinges and a water tank flew away.

Even before the storm, Puerto Rico's electrical grid was crumbling and the island was in dire condition financially.

Puerto Rico is struggling to restructure a portion of its $73-billion debt, and the government has warned it is running out of money as it fights back against furloughs and other austerity measures imposed by a federal board overseeing the island's finances.

More than 11,000 people — and more than 580 pets — were in shelters, authorities said.

'This is going to be a disaster'

Many fear extended power outages will further sink businesses struggling amid a recession that has lasted more than a decade.

"This is going to be a disaster," said Jean Robert Auguste, who owns two French restaurants and sought shelter at a San Juan hotel. "We haven't made any money this month."

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Paul Nabor, who lives in St. Thomas in the U.S. Virgin Islands, said Maria was "a lot louder and a lot wetter" than Irma.

He said water was pouring in from windows and doors, and every crack.

"It was a very scary storm," he told CBC News.

He said it's hard to reach friends and family as cell reception has been damaged. 

"I'm still waiting to hear from my roommate who made it over to Puerto Rico, only to get the eye of the storm."

Previously a Category 5 with 280 km/h winds, Maria hit Puerto Rico as the third-strongest storm to make landfall in the U.S., based on a key measurement that meteorologists use: air pressure. The lower the central pressure, the stronger a storm.

Maria's pressure was 917 millibars, lower than Hurricane Irma's 929 millibars when it roared into the Florida Keys earlier this month.

Hurricanes tend to veer north or south of Puerto Rico. The last Category 4 hurricane to blow ashore in Puerto Rico was in 1932, and the strongest ever to hit the island was San Felipe in 1928 with winds of 250 km/h.

'Grave' situation in Dominica

Maria killed two people on the French Caribbean island of Guadeloupe, and two people aboard a boat were reported missing off La Desirade island, officials said.

The storm also slammed the island of Dominica late Monday. Hartley Henry, an adviser to the prime minister, reported at least seven deaths and a "tremendous loss of housing and public buildings." He said the country was "in a daze," with no electricity and little to no communications.

"The situation is really grave," Consul General Barbara Dailey said in New York.