(CNN) There is still time for Puerto Ricans to get to a government-run shelter before powerful Hurricane Maria clobbers the island, Gov. Ricardo Rosselló told CNN late Tuesday.

But the window of opportunity is closing.

"There's still time ... but people need to move fast," Ricardo Rosselló told CNN's "Anderson Cooper 360˚," calling the storm the "worst hurricane in modern history in Puerto Rico."

US President Trump tweeted his thoughts to those on the island, a US territory. "Be careful, our hearts are with you," he wrote, calling the storm a "monster hurricane," and pledging assistance.

Puerto Rico being hit hard by new monster Hurricane. Be careful, our hearts are with you- will be there to help!

Maria is a Category 5 storm with sustained winds of 175 mph (281 kph). By late Tuesday, the eye of hurricane was expected to move near or over St. Croix in the US Virgin Islands and make landfall in Puerto Rico Wednesday morning.

"Winds are starting to pick up, the glass is starting to bow back and forth. We're soon going to take shelter in a safe room," Mike Theiss, a hurricane chaser who is staying in Fajardo, told CNN's Don Lemon.

Maria has already obliterated parts of Dominica and killed at least one person in Guadeloupe.

Rosselló said there are about 500 shelters open. Emergency workers are still out, but will head in when sustained winds reach 50 mph, the governor said.

Most ppl at Pedrin Zorria shelter will lose their home if #HurricaneMaria hits. However, mood is light & everyone helps each other here@CNN pic.twitter.com/J9zFwKcvEa — Jaide Garcia (@Jaide_Garcia) September 20, 2017

Many people live on the eastern half of Puerto Rico, which forecasters worry will endure the brunt of the most powerful hurricane ever to hit the island.

Residents of Puerto Rico, who were spared some of Irma's wrath when that hurricane's core passed north of the island, boarded up homes and businesses Tuesday. People who fled to the US territory as Maria, and before it, Hurricane Irma, which swept through the region just days ago, advanced on their islands sought refuge in hotels and shelters.

Photos: Hurricane Maria slams the Caribbean An apartment building is missing a wall in San Juan, Puerto Rico, on Monday, September 25, nearly a week after Hurricane Maria devastated the US commonwealth. Power is still out in most places, and communications remain almost nonexistent on the island of 3.4 million people. Hide Caption 1 of 36 Photos: Hurricane Maria slams the Caribbean Yancy Leon rests at the Luis Muñoz Marin International Airport near San Juan on September 25. She's been waiting in line for two days to get a flight out. Hide Caption 2 of 36 Photos: Hurricane Maria slams the Caribbean An aerial view shows the flooding in San Juan on September 25. Hide Caption 3 of 36 Photos: Hurricane Maria slams the Caribbean People collect water from a natural spring created by landslides in Corozal, Puerto Rico, on Sunday, September 24. Puerto Rican Gov. Ricardo Rosselló said the island faces a humanitarian crisis. Hide Caption 4 of 36 Photos: Hurricane Maria slams the Caribbean An aerial view shows a flooded neighborhood in Catano, Puerto Rico, on Friday, September 22. Hide Caption 5 of 36 Photos: Hurricane Maria slams the Caribbean A man cleans a muddy street in Toa Baja, Puerto Rico, on September 22. Hide Caption 6 of 36 Photos: Hurricane Maria slams the Caribbean A man walks on a highway divider while carrying his bicycle through San Juan, Puerto Rico, on Thursday, September 21. Hide Caption 7 of 36 Photos: Hurricane Maria slams the Caribbean A shack is destroyed in San Juan on September 21. Hide Caption 8 of 36 Photos: Hurricane Maria slams the Caribbean A gas station's sign is damaged in Punta Cana, Dominican Republic, as the hurricane passed just north of the country on September 21. Hide Caption 9 of 36 Photos: Hurricane Maria slams the Caribbean Rescue workers drive through a flooded road in Humacao, Puerto Rico, on Wednesday, September 20. Hide Caption 10 of 36 Photos: Hurricane Maria slams the Caribbean A mattress that fell from the third floor is surrounded by debris outside a San Juan apartment complex on September 20. Hide Caption 11 of 36 Photos: Hurricane Maria slams the Caribbean Damage is seen in Roseau, Dominica, on September 20. Hide Caption 12 of 36 Photos: Hurricane Maria slams the Caribbean People walk through the destruction in Roseau on September 20. Hide Caption 13 of 36 Photos: Hurricane Maria slams the Caribbean San Juan is shrouded in darkness after the hurricane knocked out power to the entire island of Puerto Rico. Hide Caption 14 of 36 Photos: Hurricane Maria slams the Caribbean Power lines are scattered across a road in Humacao, Puerto Rico, on September 20. Hide Caption 15 of 36 Photos: Hurricane Maria slams the Caribbean Residents move aluminum panels from an intersection in Humacao on September 20. Hide Caption 16 of 36 Photos: Hurricane Maria slams the Caribbean Rescue vehicles are trapped under an awning in Humacao on September 20. Hide Caption 17 of 36 Photos: Hurricane Maria slams the Caribbean Trees are toppled outside the Roberto Clemente Coliseum in San Juan on September 20. Hide Caption 18 of 36 Photos: Hurricane Maria slams the Caribbean Members of a rescue team embrace as they wait to help in Humacao on September 20. Hide Caption 19 of 36 Photos: Hurricane Maria slams the Caribbean A tree is damaged in Fajardo, Puerto Rico, on September 20. Hide Caption 20 of 36 Photos: Hurricane Maria slams the Caribbean Debris is strewn across a Fajardo street on September 20. Hide Caption 21 of 36 Photos: Hurricane Maria slams the Caribbean A woman closes her property in Naguabo, Puerto Rico, hours before Maria's arrival. Hide Caption 22 of 36 Photos: Hurricane Maria slams the Caribbean People take shelter at Puerto Rico's Humacao Arena on Tuesday, September 19. Hide Caption 23 of 36 Photos: Hurricane Maria slams the Caribbean Two girls play on cots at the Humacao Arena. Hide Caption 24 of 36 Photos: Hurricane Maria slams the Caribbean Waves crash in San Juan as the hurricane neared Puerto Rico on September 19. Hide Caption 25 of 36 Photos: Hurricane Maria slams the Caribbean People pray in Humacao on September 19. Hide Caption 26 of 36 Photos: Hurricane Maria slams the Caribbean A street is flooded in Pointe-a-Pitre, on the French Caribbean island of Guadeloupe, on September 19. Hide Caption 27 of 36 Photos: Hurricane Maria slams the Caribbean People stand near debris at a restaurant in Le Carbet, Martinique, on September 19. Hide Caption 28 of 36 Photos: Hurricane Maria slams the Caribbean People in Luquillo, Puerto Rico, board up windows of a business on September 19. Hide Caption 29 of 36 Photos: Hurricane Maria slams the Caribbean A boat is overturned off the shore of Sainte-Anne, Guadeloupe, on September 19. Hide Caption 30 of 36 Photos: Hurricane Maria slams the Caribbean Cars line up at a gas station in San Juan on September 19. Hide Caption 31 of 36 Photos: Hurricane Maria slams the Caribbean A motorist drives on the flooded waterfront in Fort-de-France, Martinique, on September 19. Hide Caption 32 of 36 Photos: Hurricane Maria slams the Caribbean Floodwaters surround cars in Pointe-a-Pitre on September 19. Hide Caption 33 of 36 Photos: Hurricane Maria slams the Caribbean Soldiers patrol a street in Marigot, St. Martin, as preparations were made for Maria on September 19. Hide Caption 34 of 36 Photos: Hurricane Maria slams the Caribbean People buy provisions in Petit-Bourg, Guadeloupe, as the hurricane approached on Monday, September 18. Hide Caption 35 of 36 Photos: Hurricane Maria slams the Caribbean Customers wait in line for power generators at a store in San Juan on September 18. Hide Caption 36 of 36

Fraught journey

Jessica Mangiaracina and Bob Perkins came to San Juan from St. Thomas with their two children. Irma destroyed everything around them and threw the island into chaos.

It took them six days to get off St. Thomas, they said, killing chickens for food and making their way through areas being looted.

This couple journeyed 6 days w/ 2 kids to escape the #HurricaneIrma destruction on St. Thomas...only to face #HurricaneMaria in PR. @CNN pic.twitter.com/Ct48ucGeqk — Jaide Garcia (@Jaide_Garcia) September 19, 2017

They got to Puerto Rico by catamaran, where, like many others, they tried to get a flight out with no success.

Now they are coping with the reality that they've escaped into the middle of Hurricane Maria's angry path.

The couple is among thousands of people in shelters.

When asked what their current plan is, Perkins responded, "survive."

JUST WATCHED Puerto Rico braces for direct hit from Maria Replay More Videos ... MUST WATCH Puerto Rico braces for direct hit from Maria 02:41

Heather Farrell, a visitor to the island, is on her honeymoon with her husband Luke. They were married on September 9. She says that they had tried to cut their trip short when it became apparent that they were in Maria's firing line.

"We did try to get off, as early as Saturday but all flights were either booked or canceled. We actually are on the ocean -- our room faces the ocean. It's pretty windy but there is no rain. We'll stay inside for now."

She said that hotel staff had asked that all guests that are staying at the hotel come downstairs early Wednesday morning to a safe room that they have set up for them.

"I would rather be home than here but I guess we're making the best of it," she said.

Worst storm in almost 80 years

Millions of Puerto Ricans are hoping to survive what could be the most powerful storm to ever hit the island. Maria likely will be the first Category 4 or 5 hurricane to make landfall in Puerto Rico in more than eight decades.

If the newest NHC forecast holds true, #Maria would be the strongest hurricane ever to make landfall in Puerto Rico. https://t.co/lvkxgxBGZd pic.twitter.com/pch7usfYix — CNN Weather Center (@CNNweather) September 19, 2017

"No generation has seen a hurricane like this since San Felipe II in 1928," Rosselló said earlier. "This is an unprecedented atmospheric system."

President Donald Trump said the federal government will aid Puerto Rico in the aftermath of the storm.

"Puerto Rico being hit hard by new monster Hurricane," the President tweeted. "Be careful, our hearts are with you -- will be there to help!"

Maria has already killed one man in Guadeloupe after he ignored orders to stay inside and was struck by a falling tree, the island's government said.

Man killed by falling tree in Guadeloupe

Two other people are missing after a boat sank off the coast of La Désirade, a smaller island near the mainland of Guadeloupe. The government said about 80,000 people, or 40% of the households on the island, are without power.

Hurricane Maria caused widespread flooding Tuesday in Point-A-Pitre, Guadeloupe.

The storm also caused "widespread devastation" in Dominica, Prime Minister Roosevelt Skerrit said Tuesday.

The hurricane shredded the prime minister's house overnight and left much of the island -- population 73,000 -- in ruins.

JUST WATCHED NASA watches Maria intensify from space Replay More Videos ... MUST WATCH NASA watches Maria intensify from space 00:50

Overseas, relatives were trying to reestablish contact with loved ones on the island.

Petrona Joseph, a public relations consultant who lives in Montreal, told CNN that her sister in Dominica was at home in Roseau, huddling in the bathroom with the rest of the family when they lost their roof.

They thought about trying to go to a neighbor's home but the winds were too strong. Joseph last heard from her sister by text Monday night around 11 p.m.

Maria is now the strongest hurricane on record to make landfall in Dominica, a former French and British colony whose economy relies heavily on tourism and agriculture.

Puerto Rico says Maria 'will be catastrophic'

Maria could bring up to 2 feet of rain and a storm surge of up to 9 feet to Puerto Rico, sparking flooding and mudslides, CNN Meteorologist Allison Chinchar said.

In the capital city of San Juan, residents cleared store shelves of water and other supplies.

1 San Juan store asking customers to limit to 2 cases of water per person - but shelves empty. Water easy to find in pharmacies last night. pic.twitter.com/VAmfn1fyr3 — Sarah Faidell (@FaidellCNN) September 19, 2017

A CNN producer who was driving from the airport Tuesday said there were still some vehicles on the road. Of the two gas stations she passed, one was open. Many shops were already closed, she observed.

Some tourists from California told her that the hostel they are staying at moved them to an interior room when the men were unable to find flights home.

Gov. Rosselló has declared a state of emergency, and Trump has issued an emergency declaration for Puerto Rico to aid with federal assistance.

Puerto Rico sheltered many of the evacuees who fled from other Caribbean Islands during Hurricane Irma earlier this month. Now those evacuees and native Puerto Ricans are bracing for devastation.

"This is an event that will be damaging to the infrastructure, that will be catastrophic," Rosselló said. "Our only focus right now should be to make sure we save lives."

"We expect to feel storm winds, tropical storm winds, (from) Tuesday up until late on Thursday. That's about two-and-a-half days of tropical storm winds," Rosselló said.

Our hotel in San Juan has put up metal boards to prep for #hurricanemaria pic.twitter.com/lcsmgVHphK — Julia Jones (@juliatjones) September 19, 2017

The Puerto Rico Convention Center in the capital San Juan to the north -- which is still housing Hurricane Irma evacuees from other Caribbean islands -- is preparing to accept thousands of residents as the worst of the storm is felt.

Martinique largely spared

One bit of good news emerged from the Caribbean: The French island of Martinique suffered no major damage, the French Interior Ministry tweeted Tuesday.

Maria knocked out power to about 50,000 homes, and 10,000 homes had no water. But overall, the damage assessment was "reassuring," the French Interior Ministry said.

The director general of French civil security, Jacques Witkowski, said only two people on Martinique suffered minor injuries.

Photos: Hurricane Irma tears through Caribbean Carmelo Mota, a builder, searches for tools in his destroyed home in Charlotte Amalie, US Virgin Islands, on Monday, September 18. Hurricane Irma devastated the US territory and other Caribbean islands in the region, leaving them exposed to new storms brewing in the Atlantic. Hide Caption 1 of 55 Photos: Hurricane Irma tears through Caribbean An aerial photo shows the devastation in Road Town, the capital of Tortola, the largest and most populated of the British Virgin Islands, on Wednesday, September 13. Hide Caption 2 of 55 Photos: Hurricane Irma tears through Caribbean UK Foreign Secretary Boris Johnson talks to a resident of Anguilla during a visit on September 13. Hide Caption 3 of 55 Photos: Hurricane Irma tears through Caribbean People collect food that was delivered by emergency workers in the Sandy Ground area of Marigot, St. Martin, on Tuesday, September 12. Hide Caption 4 of 55 Photos: Hurricane Irma tears through Caribbean Buildings are destroyed in St. Martin on September 12. Hide Caption 5 of 55 Photos: Hurricane Irma tears through Caribbean French President Emmanuel Macron shakes hands with St. Martin residents during a visit to the island on September 12. Hide Caption 6 of 55 Photos: Hurricane Irma tears through Caribbean French soldiers patrol St. Martin on September 12. Hide Caption 7 of 55 Photos: Hurricane Irma tears through Caribbean A person works to clean up a street September 12 after Hurricane Irma flooded parts of Havana, Cuba. Hide Caption 8 of 55 Photos: Hurricane Irma tears through Caribbean A man makes repairs in Havana on September 12. Hide Caption 9 of 55 Photos: Hurricane Irma tears through Caribbean This Marigot church was among the buildings destroyed in the storm. Hide Caption 10 of 55 Photos: Hurricane Irma tears through Caribbean Cubans affected by Hurricane Irma line up to collect drinking water in Isabela de Sagua, Cuba, on Monday, September 11. Hide Caption 11 of 55 Photos: Hurricane Irma tears through Caribbean Dutch King Willem-Alexander, front right, tours damage in St. Maarten on September 11. Hide Caption 12 of 55 Photos: Hurricane Irma tears through Caribbean A palm tree sticks out of a pool on the French side of St. Martin on September 11. Hide Caption 13 of 55 Photos: Hurricane Irma tears through Caribbean A woman stands next to her water-logged belongings that had been laid out to dry in front of her home in Isabela de Sagua on September 11. Hide Caption 14 of 55 Photos: Hurricane Irma tears through Caribbean People line up for supplies in St. Martin on September 11. Hide Caption 15 of 55 Photos: Hurricane Irma tears through Caribbean The skeleton of a boat drifts in St. Martin's Simpson Bay on September 11. Hide Caption 16 of 55 Photos: Hurricane Irma tears through Caribbean People salvage material from the remains of a house in Isabela de Sagua on September 11. Hide Caption 17 of 55 Photos: Hurricane Irma tears through Caribbean Members of the British Army provide support on Tortola, one of the British Virgin Islands. Hide Caption 18 of 55 Photos: Hurricane Irma tears through Caribbean A woman carries a dog at an airport checkpoint in St. Martin on September 11. Hide Caption 19 of 55 Photos: Hurricane Irma tears through Caribbean People wade through a flooded street as a wave crashes in Havana on Sunday, September 10. Hide Caption 20 of 55 Photos: Hurricane Irma tears through Caribbean Two men search through the rubble of their St. Martin restaurant on September 10. Hide Caption 21 of 55 Photos: Hurricane Irma tears through Caribbean People make their way through debris in the Cojimar neighborhood of Havana on September 10. Hide Caption 22 of 55 Photos: Hurricane Irma tears through Caribbean People board a plane leaving St. Martin on September 10. Hide Caption 23 of 55 Photos: Hurricane Irma tears through Caribbean A man wades through a flooded street in Havana on September 10. Hide Caption 24 of 55 Photos: Hurricane Irma tears through Caribbean An overview of Havana shows flooded streets on Saturday, September 9. Hide Caption 25 of 55 Photos: Hurricane Irma tears through Caribbean A woman surveys flooding in Havana on September 9. Hide Caption 26 of 55 Photos: Hurricane Irma tears through Caribbean A boat rests in a cemetery after Irma tore through Marigot, St. Martin. Hide Caption 27 of 55 Photos: Hurricane Irma tears through Caribbean Residents return home after Irma passed through Caibarien, Cuba, on September 9. Hide Caption 28 of 55 Photos: Hurricane Irma tears through Caribbean A man walks in Caibarien on September 9. Hide Caption 29 of 55 Photos: Hurricane Irma tears through Caribbean A man carries a child through a flooded street in Fort-Liberte, Haiti, on Friday, September 8. Hide Caption 30 of 55 Photos: Hurricane Irma tears through Caribbean A man walks on a St. Martin street covered in debris on September 8. Hide Caption 31 of 55 Photos: Hurricane Irma tears through Caribbean A damaged home is tilted onto its side on the Puerto Rican island of Culebra on Thursday, September 7. Hide Caption 32 of 55 Photos: Hurricane Irma tears through Caribbean A home is surrounded by debris in Nagua, Dominican Republic, on September 7. Hide Caption 33 of 55 Photos: Hurricane Irma tears through Caribbean Irma damage is seen in St. Martin's Orient Bay on September 7. Hide Caption 34 of 55 Photos: Hurricane Irma tears through Caribbean Employees from an electrical company work to clear a fallen tree in Sanchez, Dominican Republic. Hide Caption 35 of 55 Photos: Hurricane Irma tears through Caribbean A woman makes her way through debris in Fajardo, Puerto Rico, on September 7. Hide Caption 36 of 55 Photos: Hurricane Irma tears through Caribbean In this image made from video, damaged houses are seen in St. Thomas on September 7. Hide Caption 37 of 55 Photos: Hurricane Irma tears through Caribbean The storm left widespread destruction on the island of Barbuda on September 7. Hide Caption 38 of 55 Photos: Hurricane Irma tears through Caribbean A flattened home is seen in Nagua, Dominican Republic, on September 7. Hide Caption 39 of 55 Photos: Hurricane Irma tears through Caribbean Nagua residents ride through an area affected by the storm on September 7. Hide Caption 40 of 55 Photos: Hurricane Irma tears through Caribbean Trash and debris is washed ashore in Cap-Haitien, Haiti, on September 7. Hide Caption 41 of 55 Photos: Hurricane Irma tears through Caribbean People walk through damage in Marigot, St. Martin, on September 7. Hide Caption 42 of 55 Photos: Hurricane Irma tears through Caribbean People survey damage in Marigot on September 7. Hide Caption 43 of 55 Photos: Hurricane Irma tears through Caribbean Bluebeard's Castle, a resort in St. Thomas, was hit hard by Irma. St. Thomas resident David Velez sent this photo to CNN on September 7. Hide Caption 44 of 55 Photos: Hurricane Irma tears through Caribbean Irma ruined these vehicles in St. Thomas. Hide Caption 45 of 55 Photos: Hurricane Irma tears through Caribbean Waves smash into St. Martin on Wednesday, September 6. Hide Caption 46 of 55 Photos: Hurricane Irma tears through Caribbean A man looks at a vehicle turned upside down in the British territory of Anguilla. Hide Caption 47 of 55 Photos: Hurricane Irma tears through Caribbean An aerial view of St. Martin on September 6. Hide Caption 48 of 55 Photos: Hurricane Irma tears through Caribbean Damaged cars are seen on a St. Martin beach on September 6. Hide Caption 49 of 55 Photos: Hurricane Irma tears through Caribbean A boat is washed onto shore in St. Martin. Hide Caption 50 of 55 Photos: Hurricane Irma tears through Caribbean Cars are piled up in Marigot on September 6. Hide Caption 51 of 55 Photos: Hurricane Irma tears through Caribbean A man walks past damaged buildings in St. Martin on September 6. Hide Caption 52 of 55 Photos: Hurricane Irma tears through Caribbean A car is flipped onto its side in Marigot. Hide Caption 53 of 55 Photos: Hurricane Irma tears through Caribbean Broken palm trees are scattered on a Marigot beach on September 6. Hide Caption 54 of 55 Photos: Hurricane Irma tears through Caribbean Irma floods a beach in Marigot on September 6. Hide Caption 55 of 55

Rapid intensification

In just 30 hours, Maria's intensity exploded from 65 mph on Sunday to 160 mph by Monday night, the National Hurricane Center said.

The British Foreign Office said more than 1,300 troops are on standby, either on affected islands or in nearby locations, ready to help after Maria tears through.

One military team has been deployed to the British Virgin Islands, and a British military reconnaissance team is on standby to go to the British territory of Montserrat.

The HMS Ocean is set to arrive in the area at week's end with 60 tons of government supplies.