TROPICAL storm Isaac is set to hit New Orleans, nearly seven years to the day after Hurricane Katrina devastated the city.

Forecasters say Isaac will intensify into a Category 1 hurricane by Tuesday but will be far less powerful than Katrina in 2005.

US President Barack Obama declared a state of emergency in Louisiana, making federal funding available for emergency activities related to the storm.

Isaac, which caused 24 deaths in Haiti and the Dominican Republic at the weekend, has shifted course from Tampa, where the Republican National Convention postponed its start to Tuesday because of the storm.

Hurricane warnings extended across about 530km on Monday, from Louisiana to western Florida.

The National Hurricane Centre says winds could top about 155km/h when Isaac hits land. Katrina’s winds reached a high of more than 250km/h when it hit on August 29, 2005.

The size of the warning area and the storm’s wide bands of rain and wind prompted emergency declarations in Mississippi, Florida and Alabama as well.

Evacuations were ordered for some low-lying areas, and hurricane-tested residents were boarding up homes and stocking up on food and water.

Federal emergency management agency officials say the updated flood defences around New Orleans are equipped to handle storms stronger than Isaac. Levee failures led to the catastrophic flooding in the area after Katrina, which killed 1800.

In New Orleans, officials have no plans to order evacuations and instead have told residents to hunker down.

“It’s going to be all right,” said New Orleans Mayor Mitch Landrieu.

At 5pm on Monday (0700 AEST on Tuesday), the National Hurricane Centre reported that Isaac’s top sustained winds had reached 110km/h.

A tropical system becomes a Category 1 hurricane once winds reach 119km/h.

The storm’s centre was located about 400km southeast of the mouth of the Mississippi River and was moving northwest at 20km/h. Storm surge was considered a major threat.

If the storm hits during high tide, it could push floodwaters as deep as four metres on shore in Louisiana, Mississippi and Alabama and up to 1.8 metres in the Florida Panhandle.

The US government says 78 per cent of the oil production in the Gulf of Mexico has been halted in preparation for Isaac. The Bureau of Safety and Environmental Enforcement says about one million barrels per day of oil production had been stopped as companies evacuated 346 offshore oil and gas production platforms.

Even though the storm is moving well west of Tampa, tropical storm-force winds and heavy rains are possible because of Isaac’s large size, forecasters say.

Republicans briefly called their convention to order on Monday afternoon and then adjourned until Tuesday.

Before reaching Florida, Isaac was blamed for 19 deaths in Haiti and five more in the Dominican Republic and it downed trees and power lines in Cuba.