Miami - Thousands of people fled New Orleans and other low-lying parts of Louisiana Sunday as Hurricane Katrina, now a category four storm, barreled toward the US Gulf of Mexico coast.

A US Air Force plane flying early in the morning over the eye of the hurricane detected maximum sustained winds reaching 233 kilometers (145 miles) per hour, with stronger gusts.

The category four designation brings Katrina close to killer status as the Saffir-Simpson scale used to measure them has only five grades.

Some additional strengthening is possible today. Preparations to protect life and property should be pushed to completion.

warned the NOAA. At 0900 GMT, the center of the storm was about 443 kilometers (275 miles) southeast of the mouth of the Mississippi River, with forecasters predicting it would hit New Orleans by midday Monday. Hurricane warnings were in effect along the Louisiana coast. President George W. Bush issued a state of emergency in Louisiana, clearing the way for federal aid to those affected. Louisiana Governor Kathleen Babineaux Blanco ordered mandatory evacuations of low-lying areas.

Traffic was already backing up on many highways leading away from the coast, and authorities warned residents not to wait until the last minute to find shelter.

We want you to take this very seriously. This not a test. This is the real deal. And I don't want to panic you but I wanted to make sure you understand that there is a major hurricane that is in the Gulf of Mexico.

New Orleans Mayor Ray Nagin said. Nagin said also authorities were preparing to order an evacuation of all 485,000 residents of the low-lying city by early Sunday, and were planning to open the massive Superdome sports stadium as an emergency shelter for those who cannot get out.

About 1.3 million people live in the greater New Orleans area. Twenty-one oil rigs and platforms in the Gulf of Mexico had already been evacuated in anticipation of the hurricane, fueling concerns over oil prices, which already soared to record highs Thursday. Shell announced Friday it had evacuated the first 146 people from its offshore facilities, but stressed that this had not had an impact on production.

Katrina has already been blamed for seven deaths after it slammed ashore late Thursday in southern Florida, flooding entire neighborhoods, uprooting trees and leaving more than a million people without electricity.

Most hurricane fatalities typically occur after the storm has passed, often as a result of floods or fallen power lines.

We do not want people to let their guard down. That's when we have the greatest number of deaths

said Craig Fugate, who heads Florida's emergency operations. The storm also put a damper on the MTV Video Music Awards, causing the cancellation of glitzy concerts and parties planned ahead of the main event.

But organizers remained confident that Sunday's awards ceremony in Miami would go ahead as planned, and the event's website showed fashion shots of celebrity hosts and nominees getting soaked.