* 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 (Raises Haiti death toll to 4, updates storm position)
By Susana Ferreira
PORT-AU-PRINCE, Aug 25 (Reuters) - Tropical Storm Isaac dumped torrential rains on Haiti and flattened tent camps housing survivors of a devastating earthquake, then began an assault on eastern Cuba on Saturday.
Isaac killed at least four people in Haiti and was expected to strengthen into a hurricane before hitting the Florida Keys on Sunday and crossing into 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 Monday's start of their national convention ahead of the November presidential election.
Energy operators in the Gulf of Mexico were shutting down offshore oil and gas rigs ahead of Isaac.
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].
Isaac's rain and winds lashed Haiti's southern coast 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.
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.
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.
EMERGENCY IN FLORIDA
Damage had so far 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.
By late Saturday afternoon, Isaac's center was over eastern Cuba, 120 miles (195 km) east of Camaguey, Cuba, the U.S. National Hurricane Center said.
The storm had top winds of 60 mph (95 kph) and would become a hurricane if those swirling winds reach 74 mph (119 kph). A hurricane warning was in effect for the Florida Keys and the southwest coast of Florida.
In Haiti, authorities and U.N. troops worked to clear debris and fallen trees from roads.
The government and aid groups attempted to evacuate thousands of tent camp dwellers on Friday but many Haitians chose to remain in their flimsy, makeshift homes, apparently fearing they would be robbed.
Chan Conga, who lives in a camp known as La Piste, tried to ride out the storm in her tent. But it collapsed under Isaac's winds and driving rains, forcing her to seek refuge in an old cholera clinic packed with hundreds of other people.
"We prayed and sang all night," she said. "We asked God to protect everyone."
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.
Rising winds and waves whipped eastern Cuba on Saturday afternoon, prompting government alerts for 11 provinces. In Cuba's easternmost city of Baracoa, water crashed over the seawall and ran ankle-deep through the nearby streets.
CUBA IN STORM'S PATH
More than 1,000 people were evacuated to the homes of friends and family ahead of possible flooding, Cuban TV said.
"We fear the sea and the flooding a lot more than the rain and wind. It rains frequently here all year long, but when the sea comes in everything floods," said Baracoa housewife Yamila Sanchez.
The storm was expected to pass directly over the town of Moa, the center of the Cuban nickel-mining industry, but officials said they expected no major problems and had enough ore on hand to continue processing for up to five days. Cuba is one of the ten largest nickel producers in the world.
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 on Saturday. 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, a four-day meeting 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. (Writing by Jane Sutton; additional reporting by David Adams, Michael Connor and Kevin Gray in Miami, Irene Klotz in Cape Canaveral, Jeff Franks and Nelson Acosta in Havana, Manuel Jimenez in Santo Domingo and Erwin Seba in Houston, editing by Vicki Allen and Todd Eastham)