The now Category 5 storm pummels the tiny island nation of Dominica en route to the US Virgin Islands and possibly Puerto Rico.
The fourth major Atlantic hurricane of the year, Maria, strengthened into a Category 5 storm, dropped to 4, before roaring back to maximum strength Tuesday as it churns through the eastern Caribbean.
The storm ploughed into the small island nation of Dominica on Monday with roof-ripping fury on a collision course with the Virgin Islands, possibly on Tuesday night.
The US National Hurricane Center placed Maria about 325 kilometres (205 miles) southeast of St Croix on Tuesday at 0910 GMT (5:10pm EDT).
Hurricane Maria first reached the top of the five-step Saffir-Simpson scale on Monday night when its maximum sustained winds reached 215 kilometres per hour (160 miles per hour), with higher gusts, the NHC reported.
The centre of the storm, described by the NHC as "potentially catastrophic," was located about 72 kilometres east-southeast of Dominica as of 2345 GMT (7:45pm EDT), on a track that would put it over Puerto Rico by Wednesday, the agency said.
The storm ploughed through Dominica, an island nation of 72,000 people in the eastern Caribbean, about halfway between the French islands of Guadeloupe, to the north, and Martinique, to the south, late on Monday causing widespread devastation, Prime Minister Roosevelt Skerrit said in a Facebook post.
" So far we have lost all what money can buy and replace. My greatest fear for the morning is that we will wake to news of serious physical injury and possible deaths as a result of likely landslides triggered by persistent rains," Skerrit shared.
"I am honestly not preoccupied with physical damage at this time, because it is devastating ... indeed, mind-boggling. My focus now is in rescuing the trapped and securing medical assistance for the injured," he said.
"The winds have swept away the roofs of almost every person I have spoken to or otherwise made contact with," Skerrit said. "The roof to my own official residence was among the first to go and this apparently triggered an avalanche of torn-away roofs in the city and the countryside."
Most powerful hurricane
Maria would be the most powerful hurricane to hit Puerto Rico in 85 years, since a Category 4 storm swept the US island territory in 1932, NHC spokesman Dennis Feltgen said.
The last major hurricane to strike Puerto Rico directly was Georges, which made landfall there as a Category 3 storm, he said.
The governor of Puerto Rico, Ricardo Rossello, urged island residents in a social media advisory to brace for the storm's arrival.
"It is time to seek refuge with a family member, friend or head to a state shelter," he said.
Puerto Rico narrowly avoided a direct hit two weeks ago from Hurricane Irma, which reached a rare Category 5 status and ranked as the most powerful Atlantic storm on record before devastating several smaller islands, including the US Virgin Islands of St Thomas and St John.
Residents of some islands fled in advance of the storm.
US Virgin Islands Governor Kenneth Mapp said Maria was due to pass within 16 kilometres of the island of St Croix, which escaped the brunt of Irma's Category 5-force fury on September 6 and is home to about 55,000 year-round residents, roughly half of the entire territory's population.
The territory's two other main islands, St Thomas and St John, which lie to the north of St Croix, sustained widespread heavy damage from Irma.
Mapp warned residents not to underestimate the threat from Maria, saying, "Just remember this is a live animal."
He suggested that for residents who choose their homes over an emergency shelter during the storm, they might consider climbing into a bathtub and pulling a mattress over them to stay safe.
Beth Tamplin Jones, 45, rode out Hurricane Irma earlier this month in the pantry of a friend's house on St John.
"It was so intense," said Jones, who evacuated to Puerto Rico last week and then caught a flight to Atlanta, where she planned to remain until Maria passed.
"We're in hurricane alley, so we've had other storms, but nothing like this," Jones said, referring to Irma, which killed more than 80 people in the Caribbean and the US mainland.
"I don't think anybody's ever been hit by a storm like that. To see another one coming is just so discouraging."
Maria was expected to whip up storm surges — seawater driven ashore by wind — of up to nine feet above normal tide levels, the NHC said.
Parts of Puerto Rico could see up to 25 inches (64 cm) of rain, it said.
Puerto Rico, meanwhile, opened shelters and began to dismantle construction cranes that could be vulnerable to Maria's high winds.
Forecasters also were tracking Category 1 Hurricane Jose, which will begin gradually weakening on Wednesday. It is likely to produce dangerous surf and rip currents along the east coast of the US for several more days, the NHC said on Tuesday.
Jose is about 590 kilometres (365 miles) south of Nantucket, Massachusetts, with maximum sustained winds of 120 km/h (75 mph), the NHC added.
The center of Hurricane Jose is expected to pass to the east of the New Jersey coast on Wednesday and offshore of southeastern Massachusetts by Thursday, the Miami-based weather forecaster said.