New Orleans residents have been fleeing in their thousands after the mayor ordered the evacuation ahead of the storm's expected landfall on Monday.

Roads out of the Louisiana port have been crammed with traffic.

Mr Nagin said that the first storm winds could hit New Orleans as early as daybreak on Monday and the hurricane could reach Category Four strength.

America's homeland security chief, Michael Chertoff, said the main evacuation was going well but he warned that some people seemed determined to ride out the storm.

Predicted route of Hurricane Gustav (31 August 2008)

That strikes me as exceptionally foolish," he said and appealed for the "maximum level of co-operation from the individual public".

The evacuation comes almost exactly three years after Hurricane Katrina devastated New Orleans.

In 2005, three-quarters of the city was flooded after a storm surge breached its protective levees. More than 1,800 people died in coastal areas.

Gustav weakened to a Category Three storm (with winds of up to 125mph; 201km/h) after hitting Cuba overnight, but was expected to regain strength in the Gulf of Mexico before making landfall again.

A hurricane watch is in place from Texas along to the Alabama-Florida border.

'Big, ugly storm'

Outlining the phased, mandatory evacuation of the city, Mayor Nagin warned any potential looters that they would go straight to prison.

FLASHBACK TO KATRINA Katrina struck US Gulf Coast in August 2005 as a category three storm, killing more than 1,800 people New Orleans was 80% flooded after storm surge breached protective levees US Government was blamed for slow, botched response that exacerbated disaster Hundreds of thousands of people were displaced

Though he did not repeat his earlier forecast that Gustav would be "the storm of the century", he described it as "big and ugly".

News that it had speeded up on its route in from the Gulf of Mexico was some good news, he added.

Earlier, he warned that in the West Bank area of New Orleans, the highest levees were just eight to 10 feet, and were at risk from storm surges that could be as high as 24 feet.

The floodwalls of the Harvey Canal - a waterway designed to protect residents from surges in Lake Pontchartrain - were not yet completed, the mayor said, warning that water would "punch through holes" and flood nearby areas.

On the main roads out of New Orleans, traffic now flows in the same direction along both carriageways, carrying a stream of family vehicles to the relative safety of the north, the BBC's Kevin Connolly reports.

Many of the city's major hotels are shutting down and tourists have been advised to leave before the airport closes too, our correspondent says.

At the city's main transit terminal, reports said a line snaked for more than a mile as residents waited to board buses bound for shelters in north Louisiana and beyond.

Louisiana Governor Bobby Jindal, in New Orleans to observe the evacuation, said about 700 people an hour could be flown out of the city.

Convention dilemma

Republican Party presidential hopeful John McCain and his running mate Sarah Palin were traveling to Mississippi on Sunday to observe storm preparations.

The Republican National Convention is due to open on Monday in Minnesota and officials are considering what to do about the gathering.

Mr McCain hinted there might be changes to the tone of the event, rather than a cancellation.

The White House has confirmed that neither President George W Bush nor Vice-President Dick Cheney will attend the convention because of the hurricane.

Mr Bush is due to visit Texas to oversee hurricane emergency efforts there.

Gustav claimed the lives of more than 80 people in the Caribbean, causing widespread damage in Haiti, the Dominican Republic and Jamaica over the past week.

At least 300,000 people were evacuated in Cuba as the storm brought extensive flooding and some severe damage, but no reports of deaths.