* Hurricane warning in effect for Florida Keys
* Isaac kills four, threatens heavy flooding in Haiti
* Forecast to become Category 2 hurricane in Gulf of Mexico (Updates hurricane position, damages, add quotes, changes byline, previous Port-Au-Prince)
By Jeff Franks
HAVANA, Aug 25 (Reuters) - Tropical Storm Isaac battered eastern Cuba on Saturday on its way toward the Florida Keys after its torrential rains and gusty winds left six dead in Haiti, which is still recovering from a devastating 2010 earthquake.
Roiling seas spilled onto land along the Cuban coast, forcing the evacuation of several thousand people, while others were moved from areas along rivers ahead of possible flooding.
Up to eight inches (20 cm) of rain had fallen in some spots and more was expected as the expansive storm swept northwest en route to the Florida Keys, where hurricane warnings have been posted, and the Gulf of Mexico.
Fueled by warm Gulf waters, it was forecast to strengthen into a Category 2 hurricane with 100-mph (160-kph) winds and hit the U.S. coast somewhere between the Florida Panhandle and New Orleans at midweek.
Isaac's march toward the Gulf comes as U.S. Republicans prepare to gather in Tampa, on Florida's central Gulf Coast, for the start of their national convention this week, ahead of the November presidential election.
Energy operators in the Gulf of Mexico were shutting down offshore oil and gas rigs before Isaac arrives.
The storm could spur short-term shut-downs of 43 percent of U.S. offshore oil capacity and 38 percent of its natural gas output, according to forecasters at Weather Insight, an arm of Thomson Reuters. See a FACTBOX at: [ID: nL2E8JP1T1].
In its latest advisory, the National Hurricane Center in Miami said the storm's ill-defined center was just off Cuba's northeast coast 80 miles (130 km) east-northeast of the city of Camaguey and 375 miles (605 km) east-southeast of Key West, speeding along at 20 miles per hour (31 kph).
Maximum winds were near 60 miles per hour (96 kph), the center said.
Cuban authorities said waves up to 13 feet (4 meters) and flooding had damaged houses along the coast and winds had toppled power and phone lines in some places.
So far, no deaths or injuries had been reported, which is not unusual in Cuba where the communist government is quick to evacuate its citizens before storms.
Cuban forecasters warned that flooding could spread as Isaac was expected to hug the northern coast on its way toward Key West, which lies 90 miles (145 kms) across the Florida Straits.
Baracoa, the island's easternmost city, appeared to be the hardest hit with Cuban television reporting damages to 50 buildings and downed power and phone lines.
CRASHING WAVES, HEAVY RAINS
Waves crashing over the city's malecon, or sea wall, in combination with heavy rains, had flooded the seaside boulevard and homes and commercial buildings nearby.
"This has been terrible. The intrusions of the sea have filled up the coastal boulevard. It's raining a lot and the floods have destroyed homes and a child care center," said Baracoa resident Ricardo Alba.
"The sea is furious, truly fierce," he told Reuters by telephone.
Isaac's rain and winds lashed Haiti's southern coast earlier on Saturday, flooding parts of the capital, Port-au-Prince, and ripping through flimsy resettlement camps that house more than 350,000 survivors of the 2010 earthquake.
A 10-year-old girl was killed near Port-au-Prince when a wall fell on her and a woman in the southern coastal city of Jacmel was crushed to death when a tree fell on her house, government officials said.
Civil protection officials said the death count now totaled 6 and more than 14,000 people had been evacuated, most of them to shelters. Many main roads were blocked or impassable.
At a tent camp in the seaside slum of Cite Soleil, corrugated plastic shacks were broken apart and water gushed in.
"We had never seen anything like this. Everyone fled to the church, but I didn't want to leave my home. All my things are wet," said Edeline Trevil, 47, who survived with her cat.
"I'm cold! I've been wet since last night," the shoeless woman added.
POWER OUTAGES AND FLOODING
The storm caused power outages and flooding and blew off roofs as it moved across the hilly and severely deforested Caribbean country. Winds had died down by Saturday afternoon but forecasters said rains would continue in Haiti.
So far, damage had been less than feared, said George Ngwa, Haiti spokesman for the United Nations Office of Coordination of Humanitarian Affairs. "Fortunately there are no reports of serious damage," he said.
Flooding and mudslides were still a threat in Haiti, where many people scrape by on less than $1 a day in the poorest country in the Americas. Flooding could also reignite a cholera epidemic, which has killed more than 7,500 people in Haiti since the disease first appeared in October 2010, aid workers said.
In the Dominican Republic, Isaac felled power and phone lines and left at least a dozen towns cut off by flood waters. Nearly one million people were without power, emergency officials said.
The most severe damage was reported along the south coast, including the capital Santo Domingo.
In Florida, Governor Rick Scott declared a state of emergency, an administrative step aimed at streamlining disaster preparations.
Emergency managers urged tourists to leave the Florida Keys if they could do so safely. A single road links the chain of low-lying islands to the Florida Peninsula and the Key West airport was expected to halt flights on Saturday evening.
At Cape Canaveral on Florida's east coast, squalls from the storm delayed until next week the launch of a pair of NASA satellites to study Earth's radiation belts.
Isaac has drawn especially close scrutiny because of the Republican Party's convention, during which former Massachusetts Governor Mitt Romney will receive the party's presidential nomination.
Party officials said the convention would convene on Monday as scheduled, but then recess until Tuesday afternoon.
Hurricane Center meteorologist Matt Sardi said Tampa could be hit by coastal flooding, storm surge and driving winds and rain.
"That looks like the main threat at this point," he said. (Additional reporting by Susana Ferreira in Port-Au-Prince; Jane Sutton, David Adams, Michael Connor and Kevin Gray in Miami; Nelson Acosta and Marc Frank in Havana; Manuel Jimenez in Santo Domingo and Erwin Seba in Houston.; Editing by Todd Eastham and Christopher Wilson)