New Orleans ? The mayor of New Orleans has set up an “extremely problematic” timeline for allowing residents to return to the evacuated city, which is still threatened by a weakened levee system, a lack of drinkable water and heavily polluted floodwaters, the head of the federal relief effort said Saturday.

Coast Guard Vice Adm. Thad Allen said federal officials have worked with Mayor Ray Nagin and support his vision for repopulating the city, but he called Nagin’s idea to return up to 180,000 people to New Orleans in the next week both “extremely ambitious” and “extremely problematic.”

“Our intention is to work with the mayor … in a very frank, open and unvarnished manner,” Allen told The Associated Press in an interview at Department of Homeland Security headquarters in Baton Rouge.

Nagin has announced that the city’s Algiers section, the Garden District and the French Quarter would reopen over the next week and a half, bringing back more than one-third of the city’s half-million inhabitants.

Allen said a prime public health concern is the tap water, which in most of the city remains unfit for drinking and bathing. He said he was concerned about the difficulties of communicating the risk of using that water to people who return and might run out of the bottled water they brought along.

“The water that’s there is only good for firefighting and flushing,” he said.

Another concern, Allen said, was the risk of another storm hitting the region, threatening an already delicate levee system and possibly requiring residents to be evacuated again.

“Something less than a Category 4 storm is going to present significant issues that might require the evacuation of the general population. You want to make sure you have your arms around how you will do that,” he said.

Allen called on the mayor to be “mindful of the risks” and said he would inform Nagin of his concerns when they meet on Monday.

Allen was not the only official with doubts about the mayor’s plans.

The mayor’s homeland security director, Terry Ebbert, backed away from Nagin’s promise on Friday, saying only that the city would assess the situation in the French Quarter from “day to day.” Asked repeatedly whether that meant it could open sooner or later than Sept. 26, he declined to elaborate.

Ebbert said the city’s recovery depends on getting businesses reopened, but he said the repopulation of the city was being done “in a progressive manner” to ensure the safety and health of residents. A dusk-to-dawn curfew was planned.

Meanwhile, some business owners were being allowed back into the city Saturday to get a head start on opening the rollicking bars, stores and restaurants that keep the good times rolling in New Orleans.

But many residents, from the cast-iron balconies of the French Quarter to the white-columned mansions of the Garden District, said it will be weeks, if not months, before they are ready again for partying until dawn.

“We don’t want a bunch of tourists in here while we’re trying to get our homes together, get our businesses together,” said Sandra Cimini, whose family owns a bar on Chartres Street. “It’s not going to be walking down the street with a hurricane glass in your hand until we can get everything together.”

Traffic was already heavy at checkpoints leading into Orleans Parish, where many were turned away if they had not managed to acquire the special business permits the city was issuing by fax in recent days.

The city was relaxing requirements over the weekend so that anyone with documentation showing they had businesses in specified ZIP codes could enter. Those areas, for now, are limited to Algiers, the French Quarter, the central business district and Uptown, which includes the Garden District.

Residents who return Monday to Algiers on the west bank of the Mississippi River will return to relative normalcy. While debris from trees and roofs still litters many neighborhoods, the area never flooded, the water is clean and electricity has been restored to most places.

But on the east side, home to the sections most tourists know best, it is unclear when the water will be safe for drinking or bathing.

Until then, the bars and restaurants from Uptown to the French Quarter will have to have ice delivered. That is, if they decide to open during daylight hours.

“We stay open until 4 o’clock in the morning, so it would be a little bit weird” having to close at dusk, said Steve Bartley, who works at the Tropical Isle bar on Bourbon Street. “We’ll have to adjust the hours to how business is.”

These neighborhoods were the lucky ones. They never flooded. Still, nearly three weeks after Hurricane Katrina, about 40 percent of the Big Easy was under water.

But that is down from 80 percent after the storm, and engineers say water is dropping rapidly. While water in some low-lying areas had been as deep as 20 feet, the deepest water in the city Friday was 5 feet, exposing still more of the dead.

The death toll along the Gulf Coast rose to 816, including 579 in Louisiana.

Security will be tight in the reopened neighborhoods, with Nagin and others vowing never again to let New Orleans slip into the lawlessness that gripped the city in the days after the storm. This week, he warned potential looters that soldiers carry M-16 rifles “and they might have a few bazookas we’re saving for spec people.”