Hurricane Maria was downgraded to Category 4 and its maximum sustained winds decreased slightly to 155 mph, but the storm remained "extremely dangerous," with its core expected to reach southeastern Puerto Rico Wednesday morning, officials said.
As of 5 a.m. Wednesday, Maria was about 50 miles southeast of San Juan, Puerto Rico, and moving northwest at 10 mph, the National Hurricane Center said.
The storm's eyewall was over the island of Vieques earlier Wednesday.
As rain began lashing the U.S. territory on Tuesday afternoon, Puerto Rico's governor warned that Maria could hit "with a force and violence that we haven't seen for several generations."
"We're going to lose a lot of infrastructure in Puerto Rico," Gov. Ricardo Rossello said, adding that a likely island-wide power outage and communication blackout could last for days. "We're going to have to rebuild."
Authorities warned that people in wooden or flimsy homes should find safe shelter before the storm's expected arrival on Wednesday.
"You have to evacuate. Otherwise, you're going to die," said Hector Pesquera, the island's public safety commissioner. "I don't know how to make this any clearer."
The warnings came after Dominica Prime Minister Roosevelt Skerrit sent out a series of dramatic posts on his Facebook page as the storm blew over that tiny country late Monday — but then stopped suddenly as phone and internet connections with the country were cut.
"The winds are merciless! We shall survive by the grace of God," Skerrit wrote before communications went down.
A few minutes later, he messaged he could hear the sound of galvanized steel roofing tearing off houses on the small rugged island. He said that even his own roof had blown away.
In the last message before falling silent, he appealed for international aid: "We will need help, my friends, we will need help of all kinds."
The storm 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 around 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. "As a Category 5 it would be naive not to expect any (injuries) but I don't know how many," she said.
The island's broadcast service was also down on Tuesday and Akamai Technologies, a company that tracks the status of the internet around the world, said most of Dominica's internet service appeared to have been lost by midday. The Ross University School of Medicine in Dominica reported a widespread loss of communication on the island, and relatives of students posted messages on its Facebook page saying they had been unable to talk to their loved ones since late Monday evening as the storm approached.
Dominica is particularly vulnerable to flooding because of its steep mountains, cut through with rivers that rage even after a heavy rain. It was still recovering from Tropical Storm Erika, which killed 30 people and destroyed more than 370 homes in August 2015.
Officials on the neighboring French island of Guadeloupe reported at least one death: a person hit by a falling tree. They said two other people were reported missing after their boat sank off La Desirade island, just east of Guadeloupe.
About 40 percent of the island — 80,000 homes — were without power and flooding was reported in several communities.
Next in the storm's path was St. Croix in the U.S. Virgin Islands, where the storm passed south of late Tuesday. The island was largely spared the widespread damage caused by Hurricane Irma on the chain's St. Thomas and St. John islands just two weeks ago. There were no immediate reports of damage from Maria.
In the Puerto Rican capital, San Juan, normally crowded streets and beaches were empty by Tuesday afternoon as families heading to safe shelter packed up their cars and pets or secured windows and doors around their home to prepare for severe winds expected to lash the island for 12 to 24 hours. Nearly 2,800 people were in shelters across Puerto Rico, along with 105 pets, officials said.
"We're definitely afraid," said Erica Huber, a 33-year-old teacher from Venice, Florida, who moved to Puerto Rico a month ago with her 12-year-old daughter.
"I'm more worried about the aftermath. Is there going to be enough food and water?" she said.
In shops across the island, shelves were bare after people filled shopping carts with the limited amount of water, batteries, baby formula, milk and other items they could find.
Iris Tosado, a 64-year-old widowed housewife, scanned the nearly empty shelves before heading back home. She and her disabled son planned to spend the storm with relatives because their home is made of wood, and she prayed that it would not be destroyed.
"God, it's the only thing I have,'" she said. "This is not looking good."
Maria had maximum sustained winds of 160 mph (260 kph) late Monday when it slammed into Dominica as a Category 5 hurricane.
By late Tuesday afternoon, the U.S. National Hurricane Center in Miami said Maria's winds had intensified to 165 mph (265 kph), and that some additional strengthening was possible Tuesday evening. At 11 p.m., Maria was centered about 120 miles (280 kilometers) southeast of San Juan and was moving west-northwest at 10 mph (16 kph).
Hurricane center forecasters said it "now appears likely" that Maria will still be at Category 5 intensity when it moves over the U.S. Virgin Islands Tuesday night and Puerto Rico on Wednesday, bringing with it "life-threatening" flooding from rain and storm surge.
Forecasters said the storm surge could raise water levels by 6 to 9 feet (1.8 to 2.7 meters) near the storm's center. The storm was predicted to bring 10 to 15 inches (25 to 38 centimeters) of rain across the islands, with more in isolated areas.
To the north, Hurricane Jose stirred up dangerous surf and rip currents along the U.S. East Coast, though forecasters said the storm was unlikely to make landfall. Big waves caused by Jose swept five people off a coastal jetty in Rhode Island and they were hospitalized after being rescued.
A tropical storm warning was posted for coastal areas in Rhode Island and Massachusetts, and tropical storm watches were up for parts of New York's Long Island and Connecticut.
Coto reported from San Juan, Puerto Rico. Associated Press writers Ben Fox in Miami and Seth Borenstein in Washington contributed to this report.