Thousands of Puerto Ricans have been told to "evacuate or die" as Category 5 Hurricane Maria bears down on the island.
After wreaking widespread devastation on Dominica and leaving the small Caribbean island virtually incommunicado, Maria is cutting a deadly path west with Puerto Rico and the Virgin Islands most at risk.
Next in the storm's path was St. Croix in the US Virgin Islands, where the storm was expected to hit late on Tuesday.
Winds have strengthened to 175 mph.
"You have to evacuate. Otherwise, you're going to die," Hector Pesquera, Puerto Rico's public safety commissioner said. "I don't know how to make this any clearer."
His stark warning came as frantic preparations are being made across much of the Caribbean ahead of the arrival of the "potentially catastrophic" Category 5 hurricane.
With winds of 160 mph Maria is threatening to inflict mudslides, flash floods, life-threatening winds and storm surges throughout the region.
Several airlines have offered to cap fares for people fleeing the path of the hurricane.
The storm is following a similar path to Hurricane Irma which ripped across the Caribbean earlier this month, claiming at least 84 deaths and reducing the island of Barbuda to rubble.
Maria is the fourth major hurricane - defined as Category 3 or higher - hit the region this year, making the season the worst in many years.
"Preparations to protect life and property should be rushed to completion," the National Hurricane Centre said.
With Maria tracking northwest hurricane warnings have been issued for the US and British Virgin Islands, Puerto Rico, St Kitts, Nevis and Montserrat.
A tropical storm warning was in force for Martinique, Antigua and Barbuda, Saba, St Eustatius, St Maarten, St Lucia and Anguilla.
Maria claimed its first victim in the French territory of Guadeloupe, where two other people were missing.
It was Dominica, a mountainous island nation of 72,000, which was the first to take a battering.
The storm later 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 4 a.m. At that point, officials had learned that 70 percent of homes had lost their roofs, including her own.
"I lost everything," she said, adding there had been no word on casualties.
Further west residents of the British and US Virgin Islands are battening down the hatches ahead of Maria's arrival.
A curfew is in force on the British Virgin Islands, which has a population of just under 31,000.
"Our islands are extremely vulnerable right now," said Orlando Smith, the prime minister.
"While Hurricane Maria may not be as strong as Hurricane Irma, our present reality is also very different.
"Effects such as potential flooding and high winds that can turn debris into dangerous projectiles can have a greater and more treacherous impact for us."
A curfew was in force with Kenneth Mapp, the governor, warning that most of the islanders will be without power for weeks - and in some cases months.
Residents in vulnerable areas of Puerto Rico, which is just over 70 miles west of St Croix, were urged to seek shelter by the governor, Ricardo Rossello.
Shelves have been stripped bare as many of the 3.7 million inhabitants braced for the worst hurricane to hit Puerto Rico since 1932.