Louisiana, USA (AP) — Isaac was on the verge of ballooning into a hurricane that could flood the coasts of four states with storm surge and heavy rains on its way to New Orleans, where residents hunkered down behind levees fortified after Katrina struck seven years ago this week.

Shelters were open for those who chose to stay or missed the chance to get away before the outer bands of the large storm blow ashore ahead of a forecast landfall in southeast Louisiana tonight or early Wednesday. However, with the exception of some low-lying areas, officials had not ordered mass evacuations.

The US National Hurricane Centre in Miami predicted Isaac would power up to hurricane strength, which is measured by winds of 74 mph, later in the day. It could be at least a Category 1 hurricane by the time it's expected to reach the swampy coast of southeast Louisiana.

In Houma, a city southwest of New Orleans, people filled a municipal auditorium-turned-shelter. However, in the bayou country of Terrebonne Parish off Highway 24, storms pose a perennial dilemma for those living a hardscrabble life.

While some of the homes along Bayou Terrebonne and other nearby waterways show signs of affluence, this section of Louisiana 24 is mostly lined with trailer homes or small, often run-down houses. Staying could be dangerous, but many here who could be in harm's way have nowhere to go and little money to get there, especially given the high price of gasoline.

Monica Boudreaux lives in a trailer on low-lying land but was talking this morning with a cousin who lived closer to the bayou. They and two friends chatted as the storm approached. Boudreaux laughed when asked what she'll do if the storm hits.

"I'm surrounded by all family," she said, referring to her friends as well as her cousin. "I'll just pick up my little fat feet and run, I guess."

Forecasters warned that Isaac was a large storm whose effects could reach out 200 miles from its centre. Water may be worse than wind because the storm could push walls of water while dumping rain to flood the low-lying coast in Louisiana, Mississippi, Alabama and the Florida Panhandle.