By Scott Malone and Gabriel Stargardter

ORLANDO/MIAMI, Fla. (Reuters) - Matthew, the first major hurricane threatening a direct hit on the United States in more than 10 years, blasted the Bahamas on Thursday as it headed for Florida after killing at least 140 people in the Caribbean, mostly in Haiti.

Matthew, carrying winds of 140 mph (220 kph), pounded the northwestern part of the island chain en route to Florida's Atlantic coast, the U.S. National Hurricane Center (NHC) said.

The storm was likely to remain a Category 4 on the five-step Saffir-Simpson scale of hurricane intensity as it approached the United States, where it could either take direct aim at Florida or tear along the state's coast through Friday night, the center said, warning of "potentially disastrous impacts."

Hurricane conditions were expected in parts of Florida later on Thursday and a dangerous storm surge is expected to reach up to 11 feet (3.35 meters) along the Florida coast, Ed Rappaport, deputy director of the Miami-based NHC, said on CNN.

"What we know is that most of the lives lost in hurricanes is due to storm surge," he said.

Some 136 people were killed in Haiti, local officials said, and thousands were displaced after the storm flattened homes, uprooted trees and inundated neighborhoods earlier in the week. Four people were killed in the Dominican Republic, which neighbors Haiti.

As the storm passed near the Bahamas capital of Nassau, howling gusts of wind brought down palms and other trees and ripped shingles off the rooftops of many houses. The eye of the hurricane was later poised to move over or near Freeport in the Bahamas.

It was too soon to predict where Matthew might do the most of its damage in the United States, but the NHC's hurricane warning extended up the Atlantic coast from southern Florida through Georgia and into South Carolina. More than 12 million people in the United States were under hurricane watches and warnings, according to the Weather Channel.

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The last major hurricane, classified as a storm bearing sustained winds of more than 110 mph (177 kph), to make landfall on U.S. shores was Hurricane Wilma in 2005.

Jeff Masters, a veteran hurricane expert, said on his Weather Underground website (www.wunderground.com) that Matthew's wind threat was especially serious at Cape Canaveral, which juts into the Atlantic off central Florida.

"If Matthew does make landfall along the Florida coast, this would be the most likely spot for it. Billions of dollars of facilities and equipment are at risk at Kennedy Space Center and nearby bases, which have never before experienced a major hurricane," Masters wrote.

NASA and the U.S. Air Force, which operate the nation’s primary space launch site at Cape Canaveral, have already taken steps to safeguard personnel and equipment.

A team of 116 employees were bunkered down inside Kennedy Space Center's Launch Control Center to ride out the hurricane.

“We've had some close calls, but as far as I know it's the first time we've had the threat of a direct hit,” NASA spokesman George Diller said by email from the hurricane bunker.

ROADS FILLED WITH EVACUEES

Roads in Florida, Georgia and North and South Carolina were jammed, and gas stations and food stores ran out of supplies as the storm approached.

Florida Governor Rick Scott warned there could be "catastrophic" damage if Matthew slammed directly into the state and urged some 1.5 million people there to evacuate.

"If you're reluctant to evacuate, just think about all the people... already killed,” Scott said at a news conference. "Time is running out. This is clearly either going to have a direct hit or come right along the coast."

Scott, who activated several thousand National Guard troops to help deal with the storm, warned that millions of people were likely to be left without power.

Florida, Georgia and South Carolina opened shelters for evacuees. As of Thursday morning, more than 3,000 people were being housed in 60 shelters in Florida, Scott said.

Those three states as well as North Carolina declared states of emergency, empowering their governors to mobilize the National Guard.

President Barack Obama called the governors of the four states on Thursday to discuss preparations for the storm. He declared a state of emergency in Florida and South Carolina, a move that authorized federal agencies to coordinate disaster relief efforts.

A White House statement said he was "committed to providing necessary federal resources to help the states respond."

Hundreds of passenger flights were canceled in south Florida, and cancellations were expected to spread north in coming days along the storm's path, airlines including American Airlines, Delta Airlines and United Airlines [UALCO.UL] said.

At about 5 p.m. EDT (2100 GMT), Matthew was about 100 miles (160 km) east-southeast of Florida's West Palm Beach, the hurricane center said. It was heading northwest at about 13 mph (20 kph) and was expected to continue on this track through Thursday.

On Tuesday and Wednesday Matthew, the strongest hurricane in the Caribbean since Felix struck Central America in 2007, whipped Cuba and Haiti with 140 mph (225 kph) winds and torrential rain, pummeling towns and destroying livestock, crops and homes.

In Florida, fuel stations on Thursday posted "out of gas" signs after cars waited in long lines to fill up. At a Subco gas station in Orlando, the pumps ran dry on Wednesday afternoon.

The shop was a stopping point for coastal residents seeking shelter inland. Among them was Jonas Sylvan, 44, of Melbourne, Florida, who planned to hole up in a hotel with his wife, two daughters and dog. "We're just trying to get away from the coast," he said. "It's safer here."

Bumper-to-bumper traffic extended for more than 10 miles (16 km) on the main highway leading west to Orlando from the coast.

In the central Florida coastal city of Jupiter, people scrambled to make preparations.

"Our house is wood construction, so who knows what will happen," said Libby Valentine, 75, of Jupiter. "The whole idea is to stay safe and hope you have the grace to deal with the aftermath because you might not have a house."

(Reporting by Brendan O'Brien in Milwaukee, Neil Hartnell in Nassau, Rich McKay in Atlanta, Nick Carey in Chicago, Harriet McLeod in Charleston, S.C., Doina Chiacu in Washington, Joseph Guyler Delva in Haiti, Irene Klotz and Laila Kearney; Writing by Frances Kerry and Tom Brown; Editing by Cynthia Osterman)