Tropical Storm Isaac closed in on the U.S. Gulf of Mexico coast on Monday, triggering some mandatory evacuation orders and disrupting U.S. offshore oil production as it threatened to make landfall between Florida and Louisiana as a full-blown hurricane.

The large, slow-moving storm swiped south Florida on Sunday before moving into warm Gulf waters. It was expected to gain hurricane force by Tuesday and could reach land Tuesday night or early Wednesday, the anniversary of devastating Hurricane Katrina seven years ago.

The U.S. National Hurricane Center warned coastal residents of potentially life threatening flooding, or storm surge, from Isaac totaling up to 12 feet (3.6 meters) in some areas.

"The weather will start going downhill overnight tonight on the northern Gulf Coast," NHC director Rick Knabb told reporters on a conference call. "Wherever it is people are going to be during the storm, they need to get there tonight."

Issac could potentially take direct aim at New Orleans, which is still struggling to fully recover from Katrina which swept across the city on August 29, 2005, killing more than 1,800 people and causing billions of dollars of damage along the coast.

"It is quite ironic that we have a hurricane threatening us on the seventh anniversary of Katrina," New Orleans Mayor Mitch Landrieu told a news conference.

"That brings a high level of anxiety to the people of New Orleans," he added. "I want to tell everybody now that I believe that we will be OK."

At 2 p.m. EDT (1800 GMT), Isaac was about 280 miles (450 km) southeast of the mouth of the Mississippi River with top sustained winds of 65 mph (100 kph) and moving northwest at 14 mph (24 kph).

It was expected to be centered over the Gulf Coast no later than early on Wednesday.

The governors of Louisiana, Mississippi and Alabama all declared states of emergency on Sunday as a hurricane warning went into effect for the northern Gulf Coast from Louisiana to the Florida Panhandle.

Mandatory evacuation orders went into effect on Monday for residents of several low-lying districts outside New Orleans and its new flood-protection system. Heavily populated parts of Mobile, Alabama, were also evacuated, where city workers went door-to-door to warn residents.

Energy producers in the Gulf worked to shut down some of their operations ahead of 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.

Isaac also prompted the closure of the ports of Mobile and New Orleans on Monday and suspended all barge traffic along southern portions of the Mississippi River.

Gulf residents started stocking up on supplies and securing their homes. In New Orleans, which sits below sea level, long lines formed at some gas stations and in Gulfport, Mississippi, people crowded supermarkets to buy bottled water and canned food.

"HOPE FOR THE BEST"

A bumper-to-bumper stream of vehicles could be seen leaving New Orleans on the I-10 highway heading west toward Baton Rouge on Monday afternoon, as motorists made their way to higher ground.

At Mandina's Restaurant, a popular New Orleans eatery flooded by eight feet (2.4 meters) of water during Katrina, fourth-generation owner Cindy Mandina said she was nervous.

"We're going to hold tight and hope for the best," Mandina said, as she prepared to close up ahead of the storm. "Pre-Katrina, you'd never close, you'd stay open, maybe lose power and then reopen as soon as possible," she said.

Colonel Edward Fleming, commander of the U.S. Army Corps of Engineers overseeing New Orleans flood protection, said massive improvements to the system put the city in a far better place than it was seven years ago.

But in low-lying Placquemines Parish, which could be the first to be lashed by Isaac's winds and storm surge, workers scrambled to stack sandbags and reinforce levees.

The parish, which stretches southeast from New Orleans, is cut in two lengthwise by the Mississippi River as it flows to the Gulf of Mexico. Much of the area lies outside the greater New Orleans levee system, and construction projects to bolster protection are not yet complete.

"We signed an agreement with the (Army Corps of Engineers) 30 days ago for over a billion dollars of work," said Plaquemines Parish President Billy Nungesser. "We're really worried about the storm surge, we really need a few more years before we see an event like this."

In its advisories on Monday, the NHC said the storm was not expected to strengthen beyond Category 1, the weakest type on the five-step Saffir-Simpson scale of hurricane intensity.

But Knabb said very warm water temperatures in the Gulf, currently ranging between 86 to 87 degrees Fahrenheit (30-30.5 Celsius) could help trigger strengthening beyond Category 1.

NHC meteorologist Jessica Schauer said the hurricane warning area included "quite a few oil rigs" but not perhaps 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.

Despite the threat to offshore oil infrastructure and Louisiana refineries, U.S. crude oil future prices were down about 74 cents in afternoon trading at $95.41.

Meteorologists at Weather Insight, an arm of Thomson Reuters, estimated the storm temporarily shut down 87 percent of the U.S. offshore oil production capacity and 80 percent of the offshore 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.

Issac's westward track meant the worst of its weather would miss Tampa, Florida, where the Republican National Convention opened its four-day meeting on Monday. Official convention events were delayed until Tuesday because of the storm.

Isaac killed at least 20 people and caused significant flooding and damage in Haiti and the Dominican Republic before skirting the southern tip of Florida on Sunday.