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

The National Hurricane Center said that around 0515 GMT, a US Air Force plane flying over the eye of the hurricane detected maximum sustained winds reaching 233 kilometers (145 miles) per hour.

The designation brings Katrina close to a killer storm capable of inflicting major damage as the Saffir-Simpson scale used to measure them has only five grades.

"Some additional strengthening is possible today," the center warned in its latest advisory. "Preparations to protect life and property should be pushed to completion."

At 0600 GMT, the center of the storm was about 500 kilometers (310 miles) southeast of the mouth of the Mississippi River, with forecasters predicting it would hit southeastern 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," New Orleans Mayor Ray Nagin said. "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."

Nagin said 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 was weaker when it made landfall in heavily populated southeastern Florida on Thursday, becoming the sixth deadly hurricane to pummel the storm-weary state in slightly more than a year.

One man was found dead aboard his boat, apparently after trying to ride out the storm at sea; divers retrieved a body from a capsized houseboat; and another man's body was found floating in floodwaters.

Three people were killed by falling trees, and an elderly man died when he lost control of his vehicle and crashed into a tree.

Coast Guard crews rescued a couple and their three children whose pleasure craft was reported missing at sea during the storm.

Thousands of utility company crews battled to repair power lines after the storm knocked out power in about 1.4 million Florida homes.

Authorities warned that with downed lines lying in flooded streets, the danger was not over for residents of affected areas.

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.

burs/mk/ss

All rights reserved. © 2005 Agence France-Presse. Sections of the information displayed on this page (dispatches, photographs, logos) are protected by intellectual property rights owned by Agence France-Presse. As a consequence, you may not copy, reproduce, modify, transmit, publish, display or in any way commercially exploit any of the content of this section without the prior written consent of Agence France-Presse.