Winds packing 255 kilometres an hour were already lashing the US islands as Maria barrelled westwards, forecast to make landfall before heading to Puerto Rico.
"Preparations against life-threatening storm surge and rainfall, flooding and destructive winds should be rushed to completion," the NHC said, warning the eye of the Category Five hurricane was approaching St Croix, one of the US Virgin Islands.
"Very violent and intense right now as we have just begun to experience hurricane force winds," said 31-year-old Coral Megahy late yesterday, as she hunkered down on St Croix. "We can hear debris banging on the aluminium windows as well right now."
Arriving just as islanders are struggling to recover from devastating Hurricane Irma, which struck earlier this month, Maria claimed two victims in the French territory of Guadeloupe, where two other people were missing.
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One person was killed by a falling tree as powerful winds whipped the archipelago, authorities said, while another died on the seafront. Two more disappeared when their boat went missing in the storm.
"We have become the Irma relief hub and our brothers and sisters across the pond can't afford for us to be crippled," she said, referring to those on St Croix.
On neighbouring Dominica, premier Roosevelt Skerrit posted on Facebook that there were initial reports of "widespread devastation," with official communications to the island completely cut off in the wake of the storm.
The airport and ports have been closed on the tropical island of 72,000 people.
A similar measure was in place in the British Virgin Islands.
"Our islands are extremely vulnerable right now," the territory's premier Orlando Smith said in a statement, warning that the storm could turn debris left by Irma into dangerous projectiles.
On Puerto Rico, Governor Ricardo Rossello described Maria as "the worst storm of the last century."
Schoolteacher Noemi Aviles Rivera, 47, who experienced Hurricane Hugo in 1989 and Georges in 1998, said: "I'm not denying I'm scared. I feel worried because it's the first time I'll see a hurricane of this magnitude."
Some 40 per cent of households in the territory of 400,000 were without power.
"Everything around me is shaking," former French minister Victorin Lurel told BFMTV from his home in the south of the island.
The Dominican Republic, whose east coast was battered by Irma, ordered citizens in part of the north to evacuate ahead of Maria's arrival.
Britain, France and the Netherlands boosted resources for the Caribbean ahead of the storm, after facing accusations that they were ill-prepared for the damage done by Irma in their overseas territories.
"We are planning for the unexpected, we are planning for the worst," said Chris Austin, head of a UK military task force set up to deal with the aftermath of Irma.
France said 110 more soldiers would be deployed to the region after widespread complaints of looting and lawlessness on St Martin after last week's storm.
Irma left around 40 people dead altogether in the Caribbean before churning west and pounding Florida, where the toll of deaths linked to the hurricane rose to 58 yesterday.
The hurricane broke records when it whipped up winds of 295 kilometres per hour for more than 33 hours straight.
French President Emmanuel Macron said the lethal sequence of hurricanes was "one of the direct consequences of global warming."
Macron hosted US President Donald Trump in Paris in July when he sought to persuade the US leader to reconsider his decision to pull out of the Paris climate accord.