(ABC NEWS) -- Hurricane Matthew, the strongest Caribbean storm in almost a decade, barreled into the Bahamas today, with the coastal U.S. lying in its path.

The Category 3 hurricane will next slam the Bahamas with maximum sustained winds of up to 115 mph. By tonight or early Thursday morning, it's expected to become a Category 4 storm again, with winds up to 130 mph as it nears Florida, according to an advisory issued at 8 a.m. ET by the National Hurricane Center.

Bahamas Prime Minister Perry Christie has voiced concern about the potential impact on the archipelago off Florida's east coast, where residents braced for hurricane-force winds, a storm surge of up to 15 feet and as much as 15 inches of rain.

"We're worried because we do not control nature," Christie said Tuesday.

The storm weakened from a Category 4 hurricane after plowing into Haiti and Cuba, where it left a trail of devastation. By nightfall on Tuesday, the powerful storm was blamed for at least 11 deaths across the Caribbean, including five in Haiti, which appears to be the hardest-hit country.

According to the United Nations, more than 377,000 people were evacuated in Cuba. And in Haiti, at least 350,000 people are in need of immediate assistance following the hurricane's deadly impact. There were reports of a powerful storm surge, violent winds and widespread flooding.

“In Haiti, the government reports that a number of people have lost their lives and estimates that at least 350,000 people need immediate assistance," UN secretary-general spokesman Stéphane Dujarric said in a statement.

Hospitals there are inundated with patients and are running short of necessities, including water. Raging floodwaters severed a key bridge linking the battered southern peninsula with the rest of the impoverished Caribbean nation, raising fears that the worst of the damage has yet to be discovered.

"Haiti is facing the largest humanitarian event witnessed since the earthquake six years ago," Mourad Wahba, the U.N. secretary general's deputy special representative for Haiti, said in a statement, referring to the devastating earthquake that killed some 200,000 people in January 2010.

A hurricane warning remains in effect for Haiti, eastern Cuba and the southeastern, central and northwestern Bahamas, according to the latest weather advisory.

No other Atlantic storm on record has packed such powerful winds for such a prolonged period as Hurricane Matthew, as its slow movement and potent energy have inflicted destruction that looks to continue as it approaches the southeastern coast of the United States.

Following a briefing with his homeland security team at FEMA headquarters in Washington, D.C., today, President Obama told reporters Hurricane Matthew is “a serious storm, and we want everybody to take it seriously as well.”

"Just remember that you can always rebuild," Obama said. "You can always repair property. You cannot restore a life if it is lost and we want to make sure that we minimize any possible loss of life or risk to people in these areas."

A hurricane warning is in effect for parts of Florida, from Golden Beach north to the Flagler/Volusia county line, as well as Lake Okeechobee. The area north of Sebastian Inlet to Fernandina Beach is under a hurricane watch. A tropical storm warning is in effect from Chokoloskee to Golden Beach, the Florida Keys from Seven Mile Bridge eastward, as well as Florida Bay, according to the National Hurricane Center.

The governors of Florida, Georgia, North Carolina and South Carolina have declared states of emergency, issuing stern warnings for millions of coastal residents to prepare to evacuate as Hurricane Matthew moves northwest at about 12 mph.

Florida Gov. Rick Scott said this could be the largest evacuation ever in the Sunshine State.

Long lines at gas stations and empty shelves at grocery stores were reported in cities across the southeast U.S. as residents stocked up on supplies and prepared to flee. Many schools are also closed and people have boarded up their beach-front homes.

“The storm did slow down, and it did move somewhat, but we are not in stable territory yet," South Carolina Gov. Nikki Haley said at a news conference this morning.

As evacuations are underway in several states, FEMA is urging residents to listen closely to state and local officials’ guidance and to take evacuation orders seriously.

“Residents and visitors should take evacuation orders seriously and heed the directions of state, local and tribal officials,” FEMA Administrator W. Craig Fugate said today. “This is a major hurricane that has the potential to cause significant harm to life and property. If instructed to evacuate, don’t wait for the next forecast, evacuate.”

Hurricane Matthew's cloud shield currently spans some 700 miles, with hurricane-force winds extending outward up to 40 miles from the storm's center.

Hurricane Matthew is expected to take aim at Florida's east coast Thursday night into Friday. The current forecast projects the storm will weaken as it touches the Carolinas on Saturday before heading back out to sea Sunday night.

"The path sharply turns east and out to sea without affecting the northeast coast," Golembo said. "Let's hope this holds."

ABC News' Max Golembo, Melissa Griffin, Alex Mallin and Ben Stein, Jason Volack contributed to this report. The Associated Press also contributed to this report.

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