The first hurricane of the season claimed its first victims in Haiti where a bridge in the southwestern town of Grand Goave collapsed.
It threatened to intensify as it makes a beeline for Cuba and then landfall in the Gulf of Mexico projected Sunday or Monday, raising fears oil supplies will be disrupted by the fourth storm in as many weeks.
Dennis is a "dangerous hurricane" that looks to strengthen to near 130 mph within 12 hours, the U.S. National Hurricane Center in Miami warned. It should pass over central Cuba within 24 hours, it said.
Thunderstorms covered all the Dominican Republic, southern Haiti and northeast Jamaica. Cayman Islands and Cuba also were under hurricane warnings, including the U.S. detention camp holding some 520 terror suspects in Guantanamo Bay.
The southern Florida Keys went on hurricane warning Thursday afternoon and ordered tourists to flee, and the southern Florida peninsula was on tropical storm watch, expecting stormy conditions within 36 hours.
CBS News Correspondent Mark Strassmann reports that from Louisiana to Florida, residents are bracing for a possible hit, just days after being soaked by Cindy, which is still causing trouble all along the Eastern Seaboard. People are especially nervous in places like Pensacola, which is still rebuilding from Ivan's destruction last September.
In Haiti, wind gusts uprooted a palm tree and flung it into a mud hut, killing one person and injuring three in southern Les Cayes town, the Red Cross said. The collapsed of the bridge also killed an unknown number of people, said officials who spoke on condition of anonymity.
Rivers burst their banks in the dangerously deforested country.
In Jamaica, where a man narrowly escaped from a car swept away by fast-moving floodwaters Wednesday night, Prime Minister Percival Patterson urged people in low-lying areas to evacuate.
"Let us all work together in unity so that we will be spared the worst," he said in a national radio broadcast.
But only about 1,000 of the 2.6 million people were in shelters late afternoon, when local forecasters said the eye of the storm was passing 50 miles north of Port Antonio, on Jamaica's northeast coast.
Cuba began evacuating more than 2,500 tourists and workers from Cayo Largo del Sur, in the string of keys along the southeast, the National Information Agency reported. Livestock also was taken to higher ground.
Most tourists were taken to other hotels in Havana and Varadero beach resort in the north.
Thousands of students at government boarding schools in the southeast were being sent home.
There were no immediate plans to evacuate detainees or troops from the U.S. detention center's Camp Delta at Guantanamo Bay, on Cuba's extreme southeast end, Gen. Jay Hood said in an interview.
At the camp, about 150 yards from the ocean, troops put heavy steel shutters on sea-facing cell windows as heavy surf sent splashes of salt spray higher than the razor wire fence. Officials said Camp Delta was built to withstand winds up to 90 mph.
Dennis strengthened into a hurricane Wednesday and became a Category 3 storm Thursday afternoon — the third storm to threaten petroleum output in the Gulf of Mexico.
The Hurricane Center said the storm could strike the United States anywhere from Florida to Louisiana on Sunday or Monday. Private forecaster AccuWeather put the storm right into U.S. Gulf of Mexico oil and gas producing facilities.
Dennis came right behind Tropical Storm Cindy, which made landfall late Tuesday in Louisiana and hindered oil production and refining. On Thursday, remnants of Cindy dumped heavy rain in parts of the Carolinas, prompting flash flood and tornado watches.
Lead forecaster Martin Nelson said it was the first time the Atlantic season had four named storms this early since record-keeping began in 1851. The season runs from June 1 to Nov. 30.
Packing sustained winds near 130 mph, Dennis could dump up to 15 inches of rain over the Sierra Maestra Mountains in southeastern Cuba and up to 10 inches over Jamaica's coffee-producing Blue Mountains, according to the Hurricane Center.
At 8 p.m. EDT, the storm was centered 50 miles southeast of Cabo Cruz in southeastern Cuba. It was moving toward the northwest 15 mph, the Hurricane Center said.
Hurricane force winds extended up to 45 miles from the center and tropical storm force winds another 140 miles.
In southern Haiti, wind gusts whipped sheets of rain that flooded roads and homes with up to three feet of debris-filled water. Tin roofs torn from homes and businesses tumbled in the wind.
Floodwaters rose to waist level in an abandoned church of Les Cayes and reached toward a table where 63-year-old Eloge Larame lay down, ill. His family of five stood on chairs, their feet still in water.
U.N. mission spokesman Damian Onses-Cardona said the biggest concern was that the rains would cause landslides on denuded mountains: "People build houses on the hills, without following any specifications, and then landslides occur, like last year in Gonaives."
Last year three catastrophic hurricanes — Frances, Ivan and Jeanne — tore through the Caribbean with a collective ferocity not seen in years, causing hundreds of deaths and billions of dollars in damages.
Haiti, which began suffering from the storm early Thursday, took the deadliest hit of last year's hurricane season when Jeanne, at the time a tropical storm, triggered flooding and mudslides: 1,500 people were killed, 900 missing and presumed dead and 200,000 left homeless. Torrential rains burst river banks and irrigation canals and unleashed mudslides that destroyed thousands of acres of fertile land in Haiti.
In Jamaica, where Hurricane Ivan destroyed 8,000 homes and killed 17 people when it brushed the coastline last year, a powerline was knocked down in eastern Morant Point and blocked the main coastal road.
"It's Dennis the menace," said 34-year-old shopkeeper Wayne Brown as he raced to box up sodas, canned food and bread from his small wooden store, which was badly damaged by Ivan last September.
"It's one storm after another, I've never seen anything like it in all my years," he said.
Jamaica's RJR radio reported one man narrowly escaped from a car swept away by fast-moving flood waters Wednesday evening in eastern St. Thomas parish, where hundreds of coastal farmers and fishermen were cut off by the storm.
Jamaica closed its airports and Air Jamaica canceled its flights.