* Death toll rises to at least 339 in Haiti
* Mass evacuations in four U.S. states
* Storm's eye nears Cape Canaveral (Updates with wind speed in paragraph 2, comments from storm chaser in paragraph 4-5)
By Scott Malone and Gabriel Stargardter
ORLANDO, Fla./MIAMI, Oct 7 (Reuters) - The first major hurricane threatening a direct hit on the United States in more than 10 years lashed Florida on Friday with heavy rains and winds after killing at least 339 people in Haiti on its destructive march north through the Caribbean.
Hurricane Matthew packed gusts of 100 miles per hour (160 kph) as it tracked north-northwest along Florida's east coast, the National Hurricane Center said in an advisory. The storm's eye was 25 miles (40 km) east of Cape Canaveral, home to the nation's chief space launch site.
"We are seriously ground zero here in Cape Canaveral -- hunkered down, lights flickering, winds are crazy," said resident Sandy Wilk on Twitter.
The storm downed power lines and trees and destroyed billboards in Cape Canaveral, reported Jeff Piotrowski, a 40-year-old storm chaser from Tulsa, Oklahoma.
"The winds are ferocious right now," he said. "It's fierce."
NASA and the U.S. Air Force, which operate the Cape Canaveral launch site, took steps to safeguard personnel and equipment. A team of 116 employees was 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.
No significant damage or injuries were reported in West Palm Beach and other communities in south Florida where the storm downed trees and power lines earlier in the night, CNN and local media reported.
About 300,000 Florida households were without power, local media reported. In West Palm Beach, street lights and houses went dark and Interstate 95 was empty as the storm rolled through the community of 100,000 people.
Hurricane Matthew was carrying extremely dangerous winds of 120 mph (195 kph) on Friday, but is expected to gradually weaken during the next 48 hours, the hurricane center said.
Matthew's winds had dropped on Thursday night and into Friday morning, downgrading it to a Category 3 on the five-step Saffir-Simpson scale of hurricane intensity. It could either plow inland or tear along the Atlantic coast through Friday night, the Miami-based center said.
Few storms with winds as powerful as Matthew's have struck Florida, and the NHC warned of "potentially disastrous impacts."
The U.S. National Weather Service said the storm could be the most powerful to strike northeast Florida in 118 years.
A dangerous storm surge was 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 339 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.
Damage and potential casualties in the Bahamas were still unclear as the storm passed near the capital, Nassau, on Thursday and then out over the western end of Grand Bahama Island.
It was too soon to predict where Matthew might do the most 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.
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.
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 early on Thursday.
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.
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. Late Thursday, Obama declared an emergency in Georgia and ordered federal aid to the state.
"Hurricane Matthew is as serious as it gets. Listen to local officials, prepare, take care of each other," Obama warned people in the path of the storm in a posting on Twitter.
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 said. (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 Catherine Evans)