The latest Category 5 storm to batter the Caribbean has developed a rare and terrifying feature that makes it even more powerful.

Meteorologists say Hurricane Maria has a "pinhole eye", a term used to describe an unusually small eye of a storm.

The smaller the eye, the faster the storm spins. And the faster it spins, the more powerful the storm.

Hurricane Maria as it approaches the Lesser Antilles. Photo / AP
Hurricane Maria as it approaches the Lesser Antilles. Photo / AP

University of Miami hurricane researcher Brian McNoldy likened the effect to what happens when spinning ice skaters bring in their arms and rotate faster.

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Maria has clobbering Puerto Rico as it barrels towards the US-owned Virgin Islands after wreaking devastation on Dominica, leaving the small Caribbean island virtually incommunicado.

Puerto Rico emergency management director Abner Gomez said the island has lost all power, the Washington Post reported this morning.

"When we can get outside, we will find our island destroyed," he said.

The storm slammed through with 250km/h winds, snapping palm trees, peeling off roofs and sending debris skidding across beaches and roads.

Earlier, authorities warned residents living in wooden or flimsy homes to find safe shelter before the storm hit.

"You have to evacuate. Otherwise, you're going to die," public safety commissioner Hector Pesquera said.

"I don't know how to make this any clearer."

Maria was scheduled to pass the north coast of the Dominican Republic by Thursday morning.

St Croix was largely spared the widespread damage caused by Hurricane Irma on the chain's St Thomas and St John islands just two weeks ago.

But this time, the island would experience five hours of hurricane-force winds starting about 11pm EST, Governor Kenneth Mapp said.

"For folks in their homes, I really recommend that you not be in any kind of sleepwear," he said during a brief press conference late Tuesday.

"Make sure you have your shoes on. Make sure you have a jacket around. Something for your head in case your roof should breach. I don't really recommend you be sleeping from 11 o'clock to 4[am]. Be aware of what's going on around you."

Initial reports are of widespread devastation. So far we have lost all what money can buy and replace. My greatest fear...

Posted by Roosevelt Skerrit on Monday, 18 September 2017

The warning came after Dominica Prime Minister Roosevelt Skerrit sent out a series of dramatic posts on his Facebook page late yesterday before the monster winds cut phone and internet connections completely.

"The winds are merciless! We shall survive by the grace of God," Skerrit wrote.

A few minutes later, he messaged he could hear the sound of galvanised steel roofing tearing off houses on the small rugged island.

"My roof is gone. I am at the complete mercy of the hurricane," he wrote.

In his last message before the lines went dead, Skerrit appealed for international aid: "We will need help, my friends, we will need help of all kinds".

The storm knocked out communications for the entire country, leaving anyone outside Dominica struggling to determine the extent of damage, though it was clearly widespread. "The situation is really grave," Consul General Barbara Dailey said in a telephone interview from New York.

She said she lost contact with the island about 4am. At that point, officials had learned that 70 per cent of homes had lost their roofs, including her own.

Tool shed/dog house damaged by Hurricane Maria in Bouillante, Guadeloupe. Photo / AP
Tool shed/dog house damaged by Hurricane Maria in Bouillante, Guadeloupe. Photo / AP

"I lost everything," she said, adding there had been no word on casualties. "As a Category 5 it would be naive not to expect any but I don't know how many," she said.

The island's broadcast service was also down Tuesday and Akamai Technologies, a company that tracks the status of the internet around the world, said most of Dominica's internet service appeared to have been lost by midday.

The Ross University School of Medicine in Dominica reported a widespread loss of communication on the island, and relatives of students posted messages on its Facebook page saying they had been unable to talk to their loved ones since late Monday evening as the storm approached.

People walk by a fallen tree off the shore of Sainte-Anne on the French Caribbean island of Guadeloupe. Photo / AP
People walk by a fallen tree off the shore of Sainte-Anne on the French Caribbean island of Guadeloupe. Photo / AP

Dominica is particularly vulnerable to flooding because of its steep mountains, cut through with rivers that rage even after a heavy rain.

It was still recovering from Tropical Storm Erika, which killed 30 people and destroyed more than 370 homes in August 2015.

Officials on the neighbouring French island of Guadeloupe reported one person had been killed after being hit by a falling tree. They said two other people were reported missing after their boat sank off La Desirade island, just east of Guadeloupe.

About 40 per cent of the island - 80,000 homes - were without power and flooding was reported in several communities.

In the Puerto Rican capital, San Juan, normally crowded streets and beaches were empty by Tuesday afternoon as families heading to safe shelter packed up their cars and pets or secured windows and doors around their home to prepare for severe winds expected to lash the island for 12 to 24 hours.

Nearly 2800 people were in shelters across Puerto Rico, along with 105 pets, officials said.

"We're definitely afraid," said Erica Huber, a 33-year-old teacher from Venice, Florida, who moved to Puerto Rico a month ago with her 12-year-old daughter.

"I'm more worried about the aftermath: Is there going to be enough food and water?" she said.

In shops across the island, shelves were bare after people filled shopping carts with the limited amount of water, batteries, baby formula, milk and other items they could find.

A member of a rescue team covers his eyes as he waits to assist in the aftermath of Hurricane Maria in Humacao, Puerto Rico. Photo / AP
A member of a rescue team covers his eyes as he waits to assist in the aftermath of Hurricane Maria in Humacao, Puerto Rico. Photo / AP

Iris Tosado, a 64-year-old widowed housewife, scanned the nearly empty shelves before heading back home. She and her disabled son planned to spend the storm with relatives because their home is made of wood, and she prayed that it would not be destroyed.

"God, it's the only thing I have,"' she said. "This is not looking good."

Maria ties for the eighth strongest storm in Atlantic history, when measured by wind speed. Irma, which had 300km/h winds, ranks second.

A study presented at the 2008 American Meteorological Society meeting by scientists at Colorado State University and the National Centre for Atmospheric Research noted that tropical cyclones with pinhole eyes are associated with rapid intensification.

In their study, pinhole eyewalls were defined as being less than 10 nautical miles in diameter. Pinhole eye were found in only 10 per cent of the 99 storms examined from 1989 to 2006.

Sixty per cent of storms with a pinhole eye reached major hurricane status and were more common in the Caribbean Sea and in the Atlantic basin near the Gulf of Mexico.