Just hours after returning to what remains of their New Orleans homes, thousands of Hurricane Katrina evacuees were today told to turn back as another fierce storm roared towards the coast.

Tropical storm Rita was upgraded to a hurricane with a windspeed of 74mph (119kmh) and gusts of 104mph as it skirted the Atlantic coast of Cuba, grinding towards South Florida on a track which again threatens devastated Louisiana.

Thousands of people had just returned to the city after an all-clear at the weekend, when the storm strengthened as it passed over the warm off-shore waters this morning.

It is now predicted to reach level 3 on the five-point Saffir-Simpson scale by the time it hits the Gulf Coast at the weekend, making landfall anywhere between north-east Mexico and the swamps west of New Orleans.

Ralph Nagin, the New Orleans mayor, gave warning that repairs to protective levees which burst beneath Katrina's onslaught last month had yet to be fully repaired and retracted his homecoming invitation, which many had already criticised as premature.

"I’m encouraging people to leave. This storm, in my opinion is as dangerous as Katrina," he said. "If anyone wants to sit this storm out with our soaked levee systems, then God bless them."

Although officials said they were arranging 200 buses for an evacuation, they conceded that they do not know exactly how many people have returned to New Orleans since the floodwater receded.

Asked for precise details of the evacuation plan, Mr Nagin replied with typical bluntness: "Just tell people to run."

About 20 per cent of the city is still under water and experts have said that even if New Orleans avoids a direct strike, it is woefully unprepared to cope with another deluge of the predicted 4in to 6in of torrential rain.

Rita's projected track would first take it first over the vulnerable city of Key West at the southern tip of the Florida Keys chain of island chain.

Fierce winds and rain have already knocked out power to almost 11,000 coastal homes and those residents who had not followed the 40,000 to evacuate were urged to seek shelter.

Jeb Bush, the Governor of Florida and brother of President Bush, said: "A very serious storm is hitting our state. Now is the time to hunker down".

Schools, government buildings, the airport and and many businesses have closed down and authorities in Miami have ordered the evacuation of mobile home parks and low-lying areas.

At 8am EDT (1300BST), Rita was centered about 100 miles (160 kilometers) east-southeast of Key West. It was moving towards the coast at a steady 15 mph.

The death toll from Katrina rose to 973 yesterday, including 736 victims in Louisiana, 218 in Mississippi and 19 combined in Florida, Georgia, Alabama and Tennessee.

Lori Donaldson, who sold her home in Maryland to move to Marathon in Middle Keys told the San Jose Mercury News: "I'm concerned because of everything that happened with Katrina. I'm scared, very scared.''

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