New Orleans Mayor Ray Nagin has suspended re-entry into the city over fears tropical storm Rita could strike.

"The conditions have changed. We have another hurricane approaching us," Nagin told a news conference on Monday.

Rita is moving into the Gulf of Mexico. The storm is building in strength and may become a hurricane. While experts think Rita will hit land in Texas, others think it could hit southeastern Louisiana.

Days earlier, Nagin had OK'd the return of residents to neighborhoods that had suffered the least damage.

However, he also warned the city's pumping system hadn't been restored to full capacity and the levee system remained fragile.

Dan Hitchings of the U.S. Army Corps of Engineers told CNN that 75 millimetres of rain would cause localized flooding in New Orleans, while 75 to 150 mm could produce flooding of up to 1.2 metres in depth.

Earlier Monday, U.S. President George Bush questioned the plan to allow residents to return to New Orleans, given ongoing concerns about the threat of more flooding and safety in the city.

"The city needs to re-emerge," the president said after a meeting of his Homeland Security Council. "It's a matter of timing."

Bush mentioned the risks posed by Rita.

Medical experts have also warned that the city is not ready and say major disease risks still remain.

The doctors supported the view of Coast Guard Vice Admiral Thad Allen, head of the federal government's hurricane response, which is at odds with advice issued by Nagin.

Vice Adm. Allen says the city stills lacks basic services and doctors warn there could be another disaster if residents return to the city too soon.

"The second wave of disaster is when you welcome the people back and the infrastructure of the city is not in place," Dr. Peter Deblieux, a casualty specialist at a New Orleans hospital, told the BBC.

Vice Adm. Allen has urged Nagin not to rush people back into areas where basic services, such as drinkable water, sewage, electricity, telephone and 911, are still not in place.

Allen also expressed concerns that another storm could cause the patchwork repairs to the city's levees to fail and bring another round of flooding.

"Everybody wants the city of New Orleans to be restarted," Allen said Monday on CBS's The Early Show.

"The mayor has a vision. We agree with that vision. The discussion we're having with the mayor is over the timing of re-entry and how to do it safely."

Asked when it would be safe for people to return, he said: "We know potable water will probably be restored soon and the levees will be fixed, so that may mean days, weeks."

Joe Cappiello, vice-president of the national hospital accreditation organization, also cautioned against reopening the city, saying several hospitals  were probably damaged beyond repair, while others may try to rush back into business before conditions are safe.

Others, while rebuilding, may lose doctors and nurses to communities elsewhere.

"Essentially the health care infrastructure of New Orleans is gone -- it no longer exists,'' Cappiello, who had just completed a three-day mission to the city for the Illinois-based Joint Commission on Accreditation of Healthcare Organizations, told The Associated Press.

Although the city has more than a dozen hospitals, none has resumed normal operations.

Officials at Children's Hospital, which Nagin had hoped would be ready in time for the planned return of residents to the Uptown neighbourhood, said they may need 10 more days to prepare.

Some city residents have already returned and many said they met little resistance at checkpoints. Power is scheduled to return to the French Quarter by Friday and to Uptown by next Monday, a spokesman for the power company said.

The flooded areas of New Orleans continued shrinking over the weekend, but crews still searched by boat for the dead.

The state Department of Health and Hospitals said the hurricane death toll in Louisiana had risen to 646. The toll across the Gulf Coast was 883.

Former president Bill Clinton, speaking on ABC's This Week Sunday, directed some blame for the plight of the poor in New Orleans at the Bush administration.