Drivers and passengers wait outside their cars as traffic snarls on the interstate highway leaving downtown New Orleans August 28, 2005. (Photo: Xinhua/REUTERS)

Hurricane Katrina, one of the strongest storms ever to threaten the United States, is bearing down on the Gulf Coast region and has prompted tens of thousands of people to be evacuated.

An immediate whole-city evacuation in New Orlean, Louisiana, wasordered Sunday night as the hurricane bears down with wind revving up to around 290 km per hour and threats of a massive storm surge.

Acknowledging the fact that some people, many of them tourists, will be unable to flee the city before the eye of the storm strikes land sometime Monday, local authorities have set up 10 places as their last resort.

Describing the situation as an "once-in-a-lifetime event," New Orlean Mayor Ray Nagin said a direct hit by Katrina's storm surge will likely topple the dams that protect the city from the surrounding water of Lake Pontchartrain, the Mississippi River andmarshes.

The bowl-shaped city, which sits below sea level with 485,000 inhabitants, must pump water out even during normal times. The hurricane is now threatening electricity that runs the pumps.

US President George W. Bush urged people living in the path of Katrina to take the storm extremely seriously and follow orders to evacuate to higher ground.

A day after declaring an emergency for Louisiana, Bush declared an emergency Sunday for the state of Mississippi and pledged federal support.

Rain started falling on extreme southeastern Louisiana as the storm moved across the Gulf of Mexico toward land.

Highways in Mississippi and Louisiana have been jammed as people headed away from Katrina's expected landfall.

All lanes were limited to northbound traffic on Interstate highways 55 and 59 in the two states.

On the economic side, report said Katrina is also a "unmitigatedbad news for consumers," because it had shut down offshore production of at least one million barrels of oil daily and threatened refinery and import operations around New Orleans.

If Katrina maintains its current strength, reports said it will be the fourth Category five hurricane on record to strike the United States.

The hurricane has already claimed nine lives in the country after making landfall last week on Florida.