The category five storm has ploughed through Dominica
Caribbean countries still in the path of Hurricane Maria are bracing for a devastating natural disaster on a scale "unseen for generations".
As the category five storm ploughed through Dominica and bore down on more islands in the region, residents were urged to flee.
Puerto Rico's public safety commissioner delivered the stark message, warning the island's 3.4 million residents: "You have to evacuate. Otherwise you're going to die. I don't know how to make this any clearer."
With nearly 70,000 people still living without power after Hurricane Irma, the territory's governor said that an island-wide outage was now likely and would last for a "long time".
"This is going to impact all of Puerto Rico with a force and violence that we haven't seen for several generations," said Ricardo Rossello.
"We're going to lose a lot of infrastructure in Puerto Rico. We're going to have to rebuild."
The 175mph winds will hit the US Virgin Islands by early morning Irish time on Wednesday, before hitting Puerto Rico shortly after.
In the rest of the Caribbean, hurricane warnings have been issued to a host of other islands, including St Kitts, Nevis and Montserrat.
Tropical storm warnings have also gone out to Martinique, Antigua and Barbuda, Saba, St Eustatius, St Maarten, St Lucia and Anguilla.
And the US and British Virgin Islands, already battered by Irma, are preparing for 20 inches more rain.
University of Miami hurricane researcher Brian McNoldy said Maria was developing "the dreaded pinhole eye", a sign that it is likely to get even more fierce - like when a spinning ice skater brings their arms in and twirls faster.
Maria's eye has shrunk to a small 16km in diameter.
"You just don't see those in weaker hurricanes," Mr McNoldy said. "It's cranking up the angular momentum."
The US is also on alert for the after-effects of Hurricane Jose, which caused little additional damage in the Caribbean after Irma but has moved north across the Atlantic.
Coastal flooding has already hit North Carolina, where parts of a main highway have been closed, and areas in Rhode Island, Massachusetts, New York and Connecticut could also be affected.
Jose has brought huge waves to the US, dragging five people - who were later saved - from a pier into the sea, but is not expected to make landfall.