Havana ? Hurricane Dennis slammed Cuba’s southern coast with 135 mph winds Friday before slicing across the Caribbean’s largest island, killing at least 10 people. The powerful storm headed toward a U.S. landfall, prompting thousands to flee the Florida Keys.

Strong winds and surf buffeted the U.S. detention camp for terror suspects at Guantanamo Bay, on the island’s eastern end, where a guard tower was washed into the sea. There were no reports of casualties.

“It’s arrived, with all its diabolical force,” President Fidel Castro said on state television. He said there were 10 deaths in southeastern Cuba, but did not provide details. Another five were killed in Haiti, bringing the storm’s overall death toll to 15.

An evacuation order was in effect in the Florida Keys, where officials feared Dennis could hit the island chain over the weekend on its way to the Gulf of Mexico.

The storm’s center made landfall on central Cuba’s southern coast in the early afternoon, forcing more than 600,000 people to flee their homes to take shelter with family or friends or in government refuges, Cuban civil defense officials said.

The storm’s power mushroomed as its winds built to 150 mph late Friday morning before ebbing slightly to 135 mph, forecasters said.

At 5 p.m. EDT, hurricane-force winds extended 65 miles with tropical storm force winds stretching another 160 miles. Dennis was moving northwest near 17 mph.

UPDATES LOCATION of hurricane and stats; map shows position and strength of hurricane Dennis; 1c x 5 inches; 46.5 mm x 127 mm

Hours before Cuba, Dennis struck Haiti and killed five people as it collapsed bridges, triggered landslides, inundated homes and blocked roads with downed power lines and trees.

Although Dennis was losing some strength as it passed over Cuba, it was expected to remain a major hurricane as it emerged over the Florida Straits and into the southeastern Gulf of Mexico later Friday evening.

Forecasters predict the storm will hit the United States anywhere from Florida to Louisiana by Sunday or Monday, the fourth storm in as many weeks to disrupt oil production.

The lower Florida Keys, including Key West, were on hurricane warning, while the southern part of the state, including Miami and Naples, were on tropical storm warning.

A hurricane watch was issued for portions of the northeastern U.S. Gulf Coast from the Steinhatchee River westward to the mouth of the Pearl River.

The U.S. detention camp at Guantanamo escaped the brunt of the storm. Heaving surf tore away a lifeguard tower at Windmill Beach and winds destroyed a bus shelter and there was minor flooding.

American troops fixed metal shutters over the steel mesh windows of some prison cells overlooking the sea at Camp Delta, which is just 150 yards from the ocean.

In Haiti, young men were charging $2 to carry travelers and their possessions across the Grand Goave River, which overflowed Thursday and collapsed a 300-foot bridge, killing four people and cutting off Haiti’s southwestern peninsula. Another person was killed by a falling tree.

The hurricane center’s lead forecaster, Martin Nelson, said it was the first time the Atlantic hurricane season had four named storms this early since record-keeping began in 1851. The season runs from June 1 to Nov. 30.

Last year, four catastrophic hurricanes – Charley, 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 damage.