MIAMI — Caribbean islands pummeled by Hurricane Irma began to grapple with the monster storm’s toll on Thursday, while nervous residents of South Florida packed highways seeking safer ground amid forecasts warning that Irma posed an increasing threat to the region.

The National Hurricane Center on Thursday issued a hurricane watch for the southernmost part of Florida, the first such alert Irma has prompted in the continental United States. A storm surge watch, which warned of potentially life-threatening levels of water, was also issued for the southern part of the state, an area that includes the Florida Keys, which a day earlier had begun evacuations.

Irma’s Category 5 force pinwheeled through the Caribbean, leaving a wake of leveled neighborhoods, ravaged seafronts and at least 10 dead, according to government officials and news reports. The storm is grinding onward toward the Bahamas with winds hitting 180 mph and higher gusts registered, according to the hurricane center, which warned of storm surges capable of swallowing huge sections of the coast.

After the Bahamas, Irma’s expected path takes aim at Florida, including the ribbon of cities, dense suburbs and swampland that are home to more than 6 million people from Palm Beach to Miami-Dade counties.

Miami Beach residents brace for Hurricane Irma as the deadly storm barrels toward southern Florida. (Zoeann Murphy, Dalton Bennett/The Washington Post)

“Look at the size of this storm,” said Florida Gov. Rick Scott (R). “It’s powerful and deadly.”

He said evacuations in the state may stretch “coast to coast” from the Atlantic to the Gulf of Mexico. With potentially millions of people hitting the road to escape, Scott said emergency efforts were underway to keep fuel shipments moving to gas stations.

Increasing evacuations were getting underway across South Florida. On Thursday, Broward County began to evacuate people from the area along the Atlantic. Miami-Dade County, which had announced evacuations on Wednesday, expanded the order on Thursday to include more people across the coastal portion of the county.

In Naples, Fla., a Best Buy opened on Thursday morning to about two dozen shoppers waiting outside. Mike Ducheneau, the manager, said that the store had gotten about 20 or 30 cellphone chargers delivered the previous night. The shoppers waiting in line flooded to the cellphone section, snatching up the chargers that remained.

“They’re always in stock,” Ducheneau said. “This was an anomaly.”

By Thursday morning, as Irma’s eye was moving north off the island of Hispaniola, aid workers in Haiti — a vulnerable nation already devastated by a major earthquake in 2010 and Hurricane Matthew in 2016 — were preparing for yet another potential disaster.

Concern centered on Haiti’s flood-prone north. Haiti raised its hurricane alert level to red, its highest, and the north coast remained under a hurricane watch as the central coast faced the threat of tropical storm winds and rain.

Nevertheless, aid groups said the national hurricane response appeared to be slow. Many evacuations in the north were set to unfold as rains rapidly approached and low-quality shelters were still being finished.

School was canceled across the country as national warnings went out through social media, radio and television. In some remote towns, word to take shelter was being spread largely via local officials with bullhorns. Though Irma’s eye was on track to pass offshore, even a glancing blow could flood roads and bridges, bring mudslides and topple rickety housing, dealing yet another setback to the hemisphere’s poorest nation.

One major concern was the spread of a cholera outbreak already plaguing Haiti. In one sense, Haiti’s series of major disasters gave the nation at least one benefit: an already large presence of international aid groups. Many groups said they were poised with teams and vehicles to help bring in medical and food aid.

With Irma skirting north of Haiti, aid agencies were hoping that a normally unlucky nation could possibly catch a break, perhaps averting the worst of the storm.

The Caribbean pummeled by Hurricane Irma as Florida prepares for storm

epa06186470 Palm trees flutter due to wind and rain in San Juan, Puerto Rico, 05 September 2017. Puerto Rico is taking measures ahead of the passage of Hurricane Irma, which is on the verge of impacting the Caribbean Lesser Antilles as it reached the maximum level of category 5. EPA-EFE/Thais Llorca (Thais Llorca/Epa-Efe)

“We might get lucky, but the preparations to cope with it have been late,” said Javier Alvarez, head of emergency response for the humanitarian organization Mercy Corps. “We’re still worried.”

Irma’s strong winds and torrential rains pummeled the Dominican Republic on Thursday, damaging homes and inundating streets in the beach towns on the north coast, according to local media reports.

Among the towns pounded by the storm were Cabarete and Sosua, part of the Puerto Plata region popular with foreign tourists. More than 5,500 people in the country were evacuated in the run-up to the storm, officials said.

As Irma has churned onward, it left behind a string of once-lush islands scoured clean by the storm’s force. Aerial images released by the Dutch Defense Ministry on the island of St. Martin showed scores of homes with roofs sheared away and palm trees stripped bare.

The president of the territorial council, Daniel Gibbs, told Radio Caraibes International that St. Martin is “95 percent destroyed.” On the islands of Barbuda and Anguilla, meanwhile, at least one death was reported on each. On Puerto Rico, at least three deaths were blamed on the hurricane, said a statement from Gov. Ricardo Rosselló Nevares.

The death toll fluctuated Thursday as officials released, then revised, the number of fatalities. And with places in the Caribbean taking direct hits from Irma, the toll was expected to rise when officials are able to communicate with people on the devastated islands.

Gaston Browne, the prime minister of Antigua and Barbuda, toured the extensive damage caused by Hurricane Irma on Sept. 6. (ABS TV Antigua)

Irma’s sustained winds — which hit 185 mph on Thursday — were the strongest recorded for an Atlantic hurricane making landfall, tied with the 1935 Florida Keys hurricane. The Florida Keys are particularly vulnerable to Irma, and Monroe County, home to the Keys, began mandatory evacuations of tourists and residents alike on Wednesday.

The National Hurricane Center in Miami was remarkably calm and hushed on Thursday, considering the storm that is fast approaching. The center on the campus of Florida International University, a location that puts it much further from the coast than the old headquarters in Coral Gables, which was badly damaged during Hurricane Andrew a quarter-century ago.

The forecasters here are giving regular updates on Hurricane Irma and taking turns giving TV interviews. The place will get busier in the next few days, and on Saturday night will go into lockdown, with metal shutters enclosing the doors and everyone in for the night until the storm passes.

While forecasts have warned that South Florida could take a painful hit, what unclear is precisely where the center of the storm, and the destructive winds of the eye wall, will be when it approaches and passes through the area.

“The wild card here is the turn,” Mark DeMaria, acting deputy director of the hurricane center, said as his colleagues stared at screens showing the approaching Category 5 storm. “Anytime a hurricane makes a turn it introduces uncertainty.”

He said the models have been consistent, and changes day to day haven’t been dramatic, only on the order of 50 miles this way or that.

“But 50 miles onshore versus right of the coast makes a huge difference in impact,” he said.

Florida’s geography, its population density pattern and the track of the storm are particularly unfortunate.

“This is a large storm coming from the south. It hits the entire population of South Florida,” said Dennis Feltgen, spokesman for the hurricane center. “That’s the worst case scenario because it takes in the entire Gold Coast population and you have the greatest impact from storm surge from that direction.”

A day earlier, Brian McNoldy, a senior research associate at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and a contributor to The Washington Post’s Capital Weather Gang, had said a potential track for the storm could send it churning up the Gold Coast.

That track could send it right to McNoldy’s cubicle and on up the Gold Coast, as if the storm were trying to grind away a century of urbanization.

“That’s extremely bad,” McNoldy said. “That’s basically every East Coast Florida city. This could easily be the most expensive U.S. storm if this happens.”

Just last month, Florida marked the 25th anniversary of Hurricane Andrew, which made landfall as a Category 5 monster and tore through a stretch of what is now known as Miami-Dade County. That storm, along with the recent devastation wrought by Hurricane Harvey in Texas, has lingered in the background of the urgent warnings issued by Florida officials.

“This storm is bigger, faster and stronger than Hurricane Andrew,” Scott, the Florida governor, said Wednesday, emphasizing that even with Irma’s uncertain trajectory, officials were preparing for a direct impact.

“Do not sit and wait for the storm to come,” he said. “It is extremely dangerous and deadly and will cause devastation. Get prepared right now.”

Scott has declared a statewide emergency and warned that in addition to potentially forcing large-scale evacuations, Irma could batter areas that last year were flooded by Hurricane Matthew. States of emergency were also declared in Georgia, South Carolina and North Carolina. On Thursday, Georgia Gov. Nathan Deal (R) expanded his declaration from six coastal counties to 30 total counties, issuing a mandatory evacuation for some areas.

Officials across Florida responded to the alarming forecasts by slowly shutting down the contours of daily life. Schools closed across the state, including the University of Central Florida in Orlando, which could face punishing weather if Irma crawls up the coastline. The University of Florida, located inland in Gainesville, announced that it was canceling a football game scheduled for Saturday and would then close its campus on Sunday and Monday.

The NFL postponed the Miami Dolphins’ season opener scheduled for Sunday, while the University of Miami — the Hurricanes — scrapped a football game set for Saturday in Arkansas so the team doesn’t have to travel.

People across Florida who planned to ride out the storm were clearing store shelves of water, food and supplies, and people trying to drive north had to search for gas — and hotel rooms. Many streamed to South Florida’s airports, but found some flights canceled and many others had exorbitant ticket prices.

Meanwhile, in the Gulf of Mexico, Hurricane Katia has prompted a hurricane warning in Mexico’s Veracruz state. The National Hurricane Center said little overall motion on the storm was expected though late Thursday.

Berman reported from Washington. Anthony Faiola in Port-au-Prince, Haiti; Lindsay de Feliz in Moncion, Dominican Republic; Daniel Cassaday in San Juan, Puerto Rico; Francisco Alvarado in Key West, Fla.; Patricia Sullivan in Naples, Fla.; and Angela Fritz, Jason Samenow, Sandhya Somashekhar, Brian Murphy and Steven Mufson in Washington contributed to this report, which will be updated throughout the day.