It is the strongest storm to hit the Caribbean island since 1932
Hurricane Maria is pummelling Puerto Rico and ripping trees and infrastructure from the ground - with deafening winds and missiles crashing through the streets.
It is the strongest storm to hit the Caribbean island since 1932 - and the second monster storm to smash into the Caribbean in a fortnight, packing highly dangerous sustained winds of 155mph.
The National Hurricane Centre downgraded Maria to a category three storm on Wednesday evening, after ripped off roofs and knocked over cell towers.
It is expected to pass off the coast of the Dominican Republic late on Wednesday and Thursday.
"The wind sounds like a woman screaming at the top of her lungs!" National Geographic photographer and storm chaser Mike Theiss posted on Twitter.
The life-threatening winds are expected to batter Puerto Rico for up for 24 hours.
"This is going to be an extremely violent phenomenon," governor Ricardo Rossello said. "We have not experienced an event of this magnitude in our modern history."
On Twitter he said: “God is with us; We are stronger than any hurricane. Together we will rise.”
Resiste Puerto Rico - Dios está con nosotros; somos más fuerte que cualquier huracán. Juntos nos vamos a levantar.— Ricardo Rossello (@ricardorossello) September 20, 2017
Roofs have been flying through the streets with windows and walls smashed through. Nearly 900,000 people have already lost power in their homes.
The storm is moving northwest and its path will take it over the already devastated Virgin Islands and up through the Turks and Caicos Islands.
The area experienced the force of Hurricane Irma just two weeks ago with serious damage leaving dangerous debris in the path of this latest disaster.
As Maria approached, US President Donald Trump tweeted: "Puerto Rico being hit hard by new monster Hurricane. Be careful, our hearts are with you - will be there to help!"
Puerto Rico being hit hard by new monster Hurricane. Be careful, our hearts are with you- will be there to help!— Donald J. Trump (@realDonaldTrump) September 20, 2017
On the US Virgin Islands, governor Kenneth Mapp told people to stay alert. He said: "For folks in their homes, I really recommend that you not be in any kind of sleepwear. Make sure you have your shoes on. Make sure you have a jacket around. Something for your head in case your roof should breach."
On the island of Guadeloupe, two people were killed, floodwaters have inundated towns and almost half the population is without power.
The storm also blew over the tiny island of Dominica on Monday, killing at least seven people.
Prime Minister Roosevelt Skerrit sent out a series of dramatic posts on his Facebook page before communications were lost, saying: "The winds are merciless! We shall survive by the grace of God."
A dangerous storm surge and the tide will bring destructive waves to Caribbean islands and the threat of flooding to areas that are normally dry.
Florida is unlikely to be hit by the full force of Maria. The storm is predicted to roll northwards up the US east coast but not make landfall there. It will, however, cause dangerous swells along the Carolinas and Virginia by the weekend.