* Landfall possible on Hurricane Katrina anniversary
* States of emergency in Alabama, Louisiana and Mississippi
* US Gulf oil and gas producers shut, evacuate platforms
* US crude oil prices up 75 cents in Asian trade
* Worst of weather set to miss Republican Convention
Tropical Storm Isaac swirled into the Gulf of Mexico on Monday, threatening to disrupt U.S. offshore oil and gas supplies and strengthen into a powerful hurricane that could make landfall near Louisiana almost seven years to the day after Katrina struck.
The storm swiped south Florida on Sunday before moving into the warm Gulf waters, where it is expected to strengthen to a Category 2 hurricane and hit the Gulf Coast somewhere between Florida and Louisiana by midweek, the U.S. National Hurricane Center said.
The governors of Alabama, Louisiana and Mississippi declared states of emergency as a hurricane warning went into effect for the northern Gulf Coast from Louisiana to the Florida Panhandle.
It included New Orleans, devastated when Hurricane Katrina swept over the city on Aug. 29, 2005, killing more than 1,800 people and causing billions of dollars of damage along the coast.
"It is difficult to realize that to the day - seven years after Katrina - another hurricane is headed our way," Mississippi Governor Phil Bryant said.
"It is important for Mississippians to take this storm seriously and prepare for potential impact."
Alabama Governor Robert Bentley ordered mandatory evacuations beginning on Monday for residents in low-lying areas along the coast.
Energy producers in the Gulf worked to shut down some of their operations ahead of in what could be the biggest test for U.S. energy installations since 2008, when Hurricanes Gustav and Ike disrupted offshore oil output for months and damaged onshore natural gas processing plants, pipelines and some refineries.
Some Gulf residents started stocking up on supplies and securing their homes. In New Orleans, long lines formed at some gas stations and in Gulfport, Mississippi, people crowded supermarkets to buy bottled water and canned food.
"I sense a high level of anxiety," said New Orleans Mayor Mitch Landrieu. "The timing, as fate would have it, on the anniversary of Katrina has everybody in a state of alertness, but that is a good thing."
Isaac is forecast to become a hurricane either Monday or Tuesday. The National Hurricane Center said the storm was expected to eventually intensify to a Category 2 hurricane with extremely dangerous sustained winds of 105 miles per hour (169 kph) as it swept across the Gulf of Mexico.
Forecasters are predicting a more westward track that could bring Isaac over the heart of the U.S. offshore oil patch, which produces about 23 percent of U.S. oil output and 7 percent of its natural gas.
With the threat to offshore oil infrastructure and Louisiana refineries, U.S. crude oil prices traded up 75 cents to $96.90 a barrel in Asia early Monday.
SHUTTING OIL PRODUCTION
Meteorologists at Weather Insight, an arm of Thomson Reuters, predict the storm will spur short-term shutdowns of 85 percent of the U.S. offshore oil production capacity and 68 percent of the natural gas output.
Once ashore, the storm could wreak havoc on low-lying fuel refineries along the Gulf Coast that account for about 40 percent of U.S. refining capacity.
That could send gasoline prices spiking just ahead of the U.S. Labor Day holiday, analysts said. "It's going right in the heart of refinery row," Phil Flynn, an analyst with Price Futures Group in Chicago, said on Sunday.
London-based BP Plc, the biggest U.S. Gulf producer, said it was shutting production at all of its Gulf of Mexico oil and gas platforms and evacuating all workers on Sunday.
Late Sunday night, Isaac was about 75 miles (120 km) west-southwest of Key West at the southernmost tip of the U.S. mainland, packing top sustained winds of 65 mph (100 kph) and moving west-northwest at 14 mph (24 kph).
It was expected to approach the Gulf Coast on Tuesday.
Issac's westward track meant the worst of its weather would miss Tampa, where the Republican National Convention was expected to open its four-day meeting on Monday but official events were delayed until Tuesday because of the storm.
Tampa, located on Florida's west coast, still faces the threat of winds, heavy rains and storm surge, forecasters said.
Isaac faced favorable conditions to strengthen over the warm waters of the Gulf, increasing the possibility it could intensify beyond a Category 2 hurricane, said NHC meteorologist David Zelinsky.
Tropical-force winds from the massive storm stretched across 400 miles (644 km), with rain bands extending even further, he added.
In south Florida, winds from Isaac forced cancellations of hundreds of flights in and out of Miami, Fort Lauderdale and other south Florida airports on Sunday. Miami Mayor Carlos Gimenez reported more than 500 cancellations affecting Miami International Airport alone.
More than half of the restaurants and other businesses were shuttered in the tourist haven of Key West after many visitors heeded official warnings to head home early.
GULF COAST RESIDENTS URGED TO GET PREPARED
Isaac moved into the Gulf of Mexico after spending several days sweeping across the Caribbean.
In Haiti, Isaac added to the misery of more than 350,000 survivors of the 2010 earthquake still living in flimsy resettlement camps as water gushed into tents and corrugated plastic shacks were ripped apart by the wind.
Authorities in the impoverished nation said the storm had killed eight people, including three children.
In the Dominican Republic, officials said three people were missing, and confirmed the death of the mayor of a town near Santo Domingo, who was swept away as he tried to save another person from a flooded river.
No deaths or injuries were reported in Cuba, which got off lightly when the storm crossed its eastern flank instead of raking up the length of the island as originally predicted.
In Mississippi, Robert Latham, the director of the state's emergency management agency, urged residents to get prepared for the storm's possible arrival.
"This is important to remember, this is a huge storm," he said. "I don't have to tell you what a storm like that can do."