The state transportation department said Mississippi Highway 43 and Mississippi Highway 604, both in Hancock County, were not passable because of storm surge driven inland.
In largely abandoned Plaquemines Parish, Campbell said an 18-mile stretch along the thinly populated east bank was being overtopped by surge. The levee had not broken.
Campbell said officials believe some people may be trapped in their homes by water from the overtopped levee but were not sure how many might still be in the area. Strong wind was hampering efforts of rescuers to get into parts of the area.
She said officials expected the water to recede to the Gulf as wind direction changes with the storm's movement..
Hundreds of thousands of people were without power across the state's southern parishes, including more than 250,000 in New Orleans and its suburbs, power provider Entergy reported.
Tens of thousands of people had been told ahead of Isaac to leave low-lying areas of Mississippi and Louisiana, including 700 patients of Louisiana nursing homes. Mississippi shut down the state's 12 shorefront casinos.
The hurricane promised to lend even more solemnity to commemoration ceremonies Wednesday for Katrina's 1,800 dead in Louisiana and Mississippi, including the tolling of the bells at St. Louis Cathedral overlooking New Orleans' Jackson Square.
The storm drew intense scrutiny because of its timing __ coinciding with Katrina and the first major speeches of the Republican National Convention in Tampa, Fla., already delayed and tempered by the storm.
Isaac promises to test a New Orleans levee system bolstered by $14 billion in federal repairs and improvements after the catastrophic failures during Katrina. But in a city that has already weathered Hurricane Gustav in 2008, many had faith.